Make root_password field in google_sql_database_instance resource updatable

[modular-magician]

This PR is to fix terraform-google-modules/terraform-google-sql-db#405 and
hashicorp/terraform-provider-google#13499

Buganizer ticket : b/265377954

This PR is to make the root_password field updatable in the google_sql_database_instance resource.

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.

sql: made `root_password` field updatable in `google_sql_database_instance`

Derived from GoogleCloudPlatform/magic-modules#7136