Support Self Service Maintenance feature

[modular-magician]

Support Self Service Maintenance feature. This adds support for 2 API fields - maintenance_version and available_maintenance_version. Fixes #11881

maintenance_version feature summary:

• can't be set during creation (API will return error if it's set during creation)
• can only be set to versions in available_maintenance_version
• versions set to an older one than the current one will be ignored
• versions set to invalid newer versions will run into API errors

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: added `maintenance_version` and `available_maintenance_versions` fields to `google_sql_database_instance` resource

Derived from GoogleCloudPlatform/magic-modules#6131

[github-actions]

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