Remove legacy approval APIs

Summary

For milestone 13.0, we should remove approval related legacy APIs and other related code.

A continuation of https://gitlab.com/gitlab-org/gitlab-ee/issues/10685

Improvements

Less code to maintain

Risks

N/A

Involved components

1. API endpoints and related entities, see deprecations listed in API doc
2. evaluate on the removal of ApproverMigrateHook in Approver and ApproverGroup models.
3. other unused methods e.g. first_regular_rule, ``
4. Remove reference to Approvable and VisibleApprovable (update in 2023: I don't quite understand this anymore. Maybe the thing was already removed)