Drop remaining metrics dashboard tables

What does this MR do and why?

Drops the metrics_dashboard_annotations and metrics_users_starred_dashboards tables.

These were used in the old Metrics Dashboard feature, where was removed from the product in %16.0. So they can (and should) go!

Notable red herring:

• There are still two mutations which were once related to these tables, but the models have been removed and the mutations no longer interact with the tables at all. They're just waiting on a major release to be removed without breaking changes to the schema.

References

• Drop metrics_dashboard_annotations table to sup... (#464344 - closed)
• Monitor: Metrics Deprecation and Removal (&10107 - closed)
• https://gitlab.com/gitlab-org/gitlab/-/issues/409734+

How to set up and validate locally

Nothing to test! Just gotta get that green pipeline. 😁

MR acceptance checklist

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