Add index to ModerationReview moderated_item_type + moderated_item_id (#18799)

* Revert changes from Jeff's PR

* Add index to ModerationReview moderated_item_type + moderated_item_id

* Add to 0.41.2 instead

* Require explicit index name for createIndex

* Move 41.2 migrations to after the 41.0 migrations

* Adopt new migration numbering scheme

* Fix comments

* Fix MySQL + MariaDB insanity

* Fix ID range validation

* Actually 382 is the last legacy ID

* Improved validation and tests

* Adopt the new-new migration ID format.

* Test fixes