Remove deprecated approvals_before_merge column and API field
The `approvals_before_merge` field has been made redundant within:
- Issue: https://gitlab.com/gitlab-org/gitlab/-/issues/11132.
- Merge request: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/16121
### Steps
1. Deprecate it properly via documentation: https://gitlab.com/gitlab-org/gitlab/-/issues/352181. Currently, it's mentioned in:
- https://docs.gitlab.com/ee/api/merge_request_approvals.html#merge-request-level-mr-approvals
- https://docs.gitlab.com/ee/api/merge_request_approvals.html#change-configuration
2. Make sure that `approvals_before_merge` replacement works properly and is not used in the database!
3. Ignore the column following [the 3-release approach](https://docs.gitlab.com/ee/development/avoiding_downtime_in_migrations.html#dropping-columns): https://gitlab.com/gitlab-org/gitlab/-/issues/353094
4. Remove the column: https://gitlab.com/gitlab-org/gitlab/-/issues/353096
5. Remove the ignore rule for the column: https://gitlab.com/gitlab-org/gitlab/-/issues/353096
6. Remove `approvals_before_merge` field from public API (must be done for v5)
7. Cleanup the remaining parts of `approvals_before_merge` field usage
epic