Update documentation and add test case for data cache setting as it is available for postgres now

[modular-magician]

b/299670813
Now that data cache is supported for PostgreSQL, this PR is for updating documentation and extending test cases for PostgreSQL data cache support

Resource Reference : https://registry.terraform.io/providers/hashicorp/google/latest/docs/resources/sql_database_instance#data_cache_enabled

Release Note Template for Downstream PRs (will be copied)

note from rileykarson@: I've made the docs update an enhancement intentionally- since it'll make it clear that new functionality is available through the API, even if there was no code change in Terraform

sql: update `google_sql_database_instance` to reflect that `data_cache_enabled` works for PostgresQL now, in addition to MYSQL

Derived from GoogleCloudPlatform/magic-modules#9241

[github-actions]

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