Remove collation from update request and make collation non-updatable

[c2thorn]

Fixes hashicorp/terraform-provider-google#10959
Fixes hashicorp/terraform-provider-google#11947

update API does not accept this field at all during updates, even if it is not changing.
b/216858127

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 in `google_sql_database_instance` where updates would fail because of the `collation` field

[modular-magician]

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

Diff report:

Terraform GA: Diff ( 2 files changed, 31 insertions(+))
Terraform Beta: Diff ( 2 files changed, 31 insertions(+))
TF Validator: Diff ( 2 files changed, 3 insertions(+), 3 deletions(-))

[modular-magician]

Tests analytics

Total tests: 2111
Passed tests 1880
Skipped tests: 226
Failed tests: 5

Action taken

Triggering VCR tests in RECORDING mode for the tests that failed during VCR. Click here to see the failed tests

TestAccFirebaserulesRelease_BasicRelease|TestAccComputeInstance_soleTenantNodeAffinities|TestAccPrivatecaCertificateAuthority_subordinateCaActivatedByFirstPartyIssuerOnCreation|TestAccSqlDatabaseInstance_basicMSSQL|TestAccPrivatecaCertificateAuthority_privatecaCertificateAuthoritySubordinateExample

[modular-magician]

Tests passed during RECORDING mode:
TestAccPrivatecaCertificateAuthority_subordinateCaActivatedByFirstPartyIssuerOnCreation[view]
TestAccSqlDatabaseInstance_basicMSSQL[view]
TestAccFirebaserulesRelease_BasicRelease[view]

Tests failed during RECORDING mode:
TestAccComputeInstance_soleTenantNodeAffinities[view]
TestAccPrivatecaCertificateAuthority_privatecaCertificateAuthoritySubordinateExample[view]

Please fix these to complete your PR
View the build log or the debug log for each test

[slevenick]

This won't force recreation because it's using the same collation as the previous config, but will update another field to ensure collation isn't sent & rejected?

[c2thorn]

yep!

made sure in local testing that update request was sent here