Pause background migrations on autovacuum by default

Proposal

Enable Gitlab::Database::HealthStatus::Indicators::AutovacuumActiveOnTable by default for all instances. This indicator is sourcing data from the database that is available for all. This will help with throttling background migrations that actively update tables.

Related discussion - https://gitlab.com/gitlab-com/gl-infra/gitlab-dedicated/team/-/issues/5484#note_2274724220.