Ability to disable auto_minor_version_upgrade on RDS

In Dedicated we set rds_postgres_version to a specific minor version (e.g. 12.10) because we needed (wanted?) to be more precise than just specifying the major version (12), so we could perform specific upgrades using just an apply of terraform, rather than doing it as an external task using the AWS CLI or web UI.

However, auto_minor_version_upgrade defaults to true on an aws_db_instance, so at any maintenance window AWS can upgrade to something newer, like 12.11, and then the next time we run it terraform fails (exit code 1) because it (quite reasonably) refuses to downgrade to the specifically requested version.

If the idea is acceptable, I'm happy to create an MR to make auto_minor_version_upgrade configurable (default to the current value of true of course, but allowing some users of GET to set it to false in certain circumstances).

@grantyoung WDYT?