Switch from an explicit default to O+C for redis readReplicasMode

[rileykarson]

Fixes hashicorp/terraform-provider-google#11410

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)

redis: fixed an issue where older redis instances had a dangerous diff on the field `read_replicas_mode`, adding a default of `READ_REPLICAS_DISABLED`. Now, if the field is not set in config, the value of the field will keep the old value from state.

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

[melinath]

LGTM. For our future reference:

• This default was originally added in Redis MRR Support #5449 as a beta-only field
• It was promoted to GA in GA several redis fields #5838
• This implementation matches changes proposed in Add support to enable read replicas on existing redis instances #5793 (which also contains changes to a related field) which were going to be made to prevent a similar issue in beta.

[modular-magician]

Tests count:
Total tests: 1966
Passed tests 1715
Skipped tests: 237
Failed tests: 14

[modular-magician]

I have triggered VCR tests in RECORDING mode for the following tests that failed during VCR: TestAccAccessApprovalSettings|TestAccAccessContextManager|TestAccContainerCluster_withAuthenticatorGroupsConfig|TestAccRedisInstance_downgradeRedisVersion|TestAccRedisInstance_redisInstanceAuthEnabled|TestAccRedisInstance_regionFromLocation|TestAccServiceNetworkingPeeredDNSDomain_basic|TestAccComputeForwardingRule_update|TestAccFirebaserulesRelease_BasicRelease|TestAccCGCSnippet_flaskGoogleCloudQuickstartExample|TestAccRedisInstance_redisInstanceFullExample|TestAccRedisInstance_redisInstanceBasicExample|TestAccRedisInstance_update|TestAccRedisInstanceDatasource_basic

[modular-magician]

Tests passed during RECORDING mode:
TestAccAccessApprovalSettings
TestAccRedisInstanceDatasource_basic
TestAccRedisInstance_redisInstanceFullExample
TestAccRedisInstance_downgradeRedisVersion
TestAccRedisInstance_redisInstanceAuthEnabled
TestAccRedisInstance_regionFromLocation
TestAccRedisInstance_update
TestAccFirebaserulesRelease_BasicRelease
TestAccRedisInstance_redisInstanceBasicExample
TestAccServiceNetworkingPeeredDNSDomain_basic
TestAccComputeForwardingRule_update
TestAccCGCSnippet_flaskGoogleCloudQuickstartExample

Tests failed during RECORDING mode:
TestAccAccessContextManager
TestAccContainerCluster_withAuthenticatorGroupsConfig

Please fix these to complete your PR
You can view the build log here: https://storage.cloud.google.com/vcr-test-logs/beta/refs/heads/auto-pr-5899/artifacts/330fe68a-e3b1-46a3-95f6-d7a3fdf5d69f/build-log/recording_test.log and the debug log for each test here: https://console.cloud.google.com/storage/browser/vcr-test-logs/beta/refs/heads/auto-pr-5899/artifacts/330fe68a-e3b1-46a3-95f6-d7a3fdf5d69f/recording