Fix edition downgrade failure for an ENTERPRISE_PLUS instance with da…

[skysarthak]

Recently there was an API change to enable data cache by default for ENTERPRISE_PLUS edition (both MySQL and Postgres) instances. When data_cache_config was not a computed attribute this resulted in acceptance test failures that were fixed in #12096.

Now that data_cache_config is computed, it stays in the resource state after a downgrade even though data_cache_config is cleared from the instance metadata. This is a consequence of performing a PATCH (for edition upgrade / downgrade) followed by an UPDATE while ignoring the change in CloudSQL instance settings from the PATCH operation. The downgrade implicitly unsets the data cache config in the API as it is not allowed to be used with ENTERPRISE edition.

This PR fixes this issue (edition downgrade failure for an ENTERPRISE_PLUS instance with data cache enabled) by persisting the data_cache_config received after the PATCH operation for edition upgrade / downgrade. Also fixes hashicorp/terraform-provider-google#20010 as TestAccSqlDatabaseInstance_Edition_Downgrade can now run.

b/375381759

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).
• 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).
• Generated Terraform providers, and ran make test and make lint in the generated providers to ensure it passes unit and linter tests.
• Ran relevant acceptance tests using my own Google Cloud project and credentials (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 edition downgrade failure for an `ENTERPRISE_PLUS` instance with data cache enabled.

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