Make sql db instance recreate when going private -> public

[c2thorn]

fixes hashicorp/terraform-provider-google#2964 (comment)

If this PR is for Terraform, I acknowledge that I have:

• Searched through the issue tracker for an open issue that this either resolves or contributes to, commented on it to claim it, and written "fixes {url}" or "part of {url}" in this PR description. If there were no relevant open issues, I opened one and commented that I would like to work on it (not necessary for very small changes).
• Generated Terraform, and ran make test and make lint to ensure it passes unit and linter tests.
• Ensured that all new fields I added that can be set by a user appear in at least one example (for generated resources) or third_party test (for handwritten resources or update tests).
• Ran relevant acceptance tests (If the acceptance tests do not yet pass or you are unable to run them, please let your reviewer know).
• Read the Release Notes Guide before writing my release note below.

Release Note Template for Downstream PRs (will be copied)

sql: fixed an issue where `google_sql_database_instance` would throw an error when removing `private_network`. Removing `private_network` now recreates the resource.

[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform GA: Diff ( 1 file changed, 14 insertions(+), 1 deletion(-))
Terraform Beta: Diff ( 1 file changed, 14 insertions(+), 1 deletion(-))

[modular-magician]

I have triggered VCR tests based on this PR's diffs. See the results here: "https://ci-oss.hashicorp.engineering/viewQueued.html?itemId=140688"

[modular-magician]

I have triggered VCR tests in RECORDING mode for the following tests that failed during VCR: TestAccDataSourceGoogleBigqueryDefaultServiceAccount_basic|TestAccDataSourceDNSKeys_basic|TestAccDataSourceDnsManagedZone_basic|TestAccDataSourceDNSKeys_noDnsSec|TestAccDataSourceComputeNetworkEndpointGroup|TestAccDataSourceGoogleComputeDefaultServiceAccount_basic|TestAccDataSourceComputeImage|TestAccDataSourceGoogleComputeInstanceGroup_basic|TestAccDataSourceGoogleComputeInstanceGroup_fromIGM|TestAccDataSourceGoogleComputeInstanceGroup_withNamedPort|TestAccDataSourceGoogleCloudFunctionsFunction_basic|TestAccDataSourceComputeAddress|TestAccDataSourceComputeBackendService_basic|TestAccDataSourceComputeBackendBucket_basic|TestAccDataSourceGoogleForwardingRule|TestAccDataSourceComputeGlobalAddress|TestAccDataSourceComputeNodeTypes_basic|TestAccComputeRegions_basic|TestAccComputeZones_basic|TestAccDataSourceGoogleNetwork|TestAccDataSourceComputeInstanceSerialPort_basic|TestAccDataSourceComputeInstance_basic|TestAccDataSourceComputeResourcePolicy|TestAccDataSourceGoogleSslPolicy|TestAccDataSourceComputeRouter|TestAccDataSourceComputeSslCertificate|TestAccDataSourceGoogleSubnetwork|TestAccDataSourceGoogleVpnGateway|TestAccContainerClusterDatasource_zonal|TestAccContainerClusterDatasource_regional|TestAccDataSourceGoogleStorageProjectServiceAccount_basic|TestAccBigQueryTableIamBindingGenerated|TestAccBigQueryTableIamMemberGenerated|TestAccBigQueryTableIamPolicyGenerated|TestAccBigQueryTableIamBindingGenerated_withCondition|TestAccBigQueryTableIamMemberGenerated_withCondition|TestAccBigQueryTableIamPolicyGenerated_withCondition|TestAccCloudFunctionsCloudFunctionIamBindingGenerated|TestAccCloudFunctionsCloudFunctionIamMemberGenerated|TestAccCloudFunctionsCloudFunctionIamPolicyGenerated|TestAccComputeInstanceIamBindingGenerated|TestAccComputeInstanceIamMemberGenerated|TestAccComputeInstanceIamBindingGenerated_withCondition|TestAccComputeInstanceIamMemberGenerated_withCondition|TestAccComputeSubnetworkIamBindingGenerated|TestAccComputeInstanceIamPolicyGenerated|TestAccComputeSubnetworkIamMemberGenerated|TestAccComputeInstanceIamPolicyGenerated_withCondition|TestAccComputeSubnetworkIamBindingGenerated_withCondition|TestAccComputeSubnetworkIamMemberGenerated_withCondition|TestAccComputeSubnetworkIamPolicyGenerated|TestAccComputeSubnetworkIamPolicyGenerated_withCondition|TestAccIapAppEngineVersionIamBindingGenerated|TestAccIapAppEngineVersionIamMemberGenerated|TestAccIapAppEngineVersionIamBindingGenerated_withCondition|TestAccIapAppEngineVersionIamPolicyGenerated|TestAccIapAppEngineVersionIamMemberGenerated_withCondition|TestAccIapTunnelInstanceIamBindingGenerated|TestAccIapAppEngineVersionIamPolicyGenerated_withCondition|TestAccIapTunnelInstanceIamMemberGenerated|TestAccIapTunnelInstanceIamBindingGenerated_withCondition|TestAccIapTunnelInstanceIamMemberGenerated_withCondition|TestAccIapTunnelInstanceIamPolicyGenerated|TestAccIapWebBackendServiceIamBindingGenerated|TestAccIapTunnelInstanceIamPolicyGenerated_withCondition|TestAccIapWebBackendServiceIamMemberGenerated|TestAccIapWebBackendServiceIamBindingGenerated_withCondition|TestAccIapWebBackendServiceIamMemberGenerated_withCondition|TestAccIapWebBackendServiceIamPolicyGenerated|TestAccIapWebBackendServiceIamPolicyGenerated_withCondition|TestAccIapAppEngineServiceIamBindingGenerated|TestAccIapAppEngineServiceIamMemberGenerated|TestAccIapAppEngineServiceIamBindingGenerated_withCondition|TestAccIapAppEngineServiceIamPolicyGenerated|TestAccIapAppEngineServiceIamMemberGenerated_withCondition|TestAccIapAppEngineServiceIamPolicyGenerated_withCondition|TestAccStorageBucketIamBindingGenerated|TestAccStorageBucketIamMemberGenerated|TestAccStorageBucketIamBindingGenerated_withCondition|TestAccStorageBucketIamMemberGenerated_withCondition|TestAccActiveDirectoryDomain_activeDirectoryDomainBasicExample|TestAccAppEngineApplicationUrlDispatchRules_appEngineApplicationUrlDispatchRulesBasicExample|TestAccAppEngineServiceSplitTraffic_appEngineServiceSplitTrafficExample|TestAccAppEngineStandardAppVersion_appEngineStandardAppVersionExample|TestAccBigQueryDataset_basic|TestAccBigQueryDataset_bigqueryDatasetBasicExample|TestAccBigQueryDataset_datasetWithContents|TestAccBigQueryDataset_access|TestAccBigQueryDataset_regionalLocation|TestAccBigQueryJob_bigqueryJobQueryExample|TestAccBigQueryJob_bigqueryJobQueryTableReferenceExample|TestAccBigQueryJob_bigqueryJobLoadExample|TestAccBigQueryJob_bigqueryJobLoadTableReferenceExample|TestAccBigQueryJob_bigqueryJobExtractExample|TestAccBigQueryJob_bigqueryJobExtractTableReferenceExample|TestAccBigQueryDatasetAccess_view|TestAccBigQueryDatasetAccess_iamMember|TestAccBigQueryDatasetAccess_allUsers|TestAccBigQueryDatasetAccess_allAuthenticatedUsers|TestAccBigQueryDatasetAccess_basic You can view the result here: "https://ci-oss.hashicorp.engineering/viewQueued.html?itemId=140691"

[rileykarson]

We'd normally need to check if the new value was known or unknown before knowing if this comparison was safe, but it's unlikely that it would be unknown given that this is a block (and the field itself isn't one we expect to be interpolated).