Remove insightsPostgresOnlyCustomizeDiff function

[modular-magician]

Hi! 👋 We've recently enabled Cloud SQL Insights on our MySQL instances, but unfortunately we're no longer able to run plan or apply in the Terraform environments which manage the instances because the insightsPostgresOnlyCustomizeDiff function that is used by the google_sql_database_instance resource enforces that the instance that Query Insights is enabled on is a Postgres instance. Would it make sense to remove this function now that the feature is available for MySQL?

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: removed requirement that Cloud SQL Insight is only allowed for Postgres in `google_sql_database_instance`

Derived from GoogleCloudPlatform/magic-modules#6006

[github-actions]

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