MR approvals should stop overriding the project setting upon disabling overrides

The following discussion from !10766 (merged) should be addressed:

• @mkozono started a discussion: (+1 comment)

  Should approvals_required decrease to the project setting when disable_overriding_approvers_per_merge_request changes from false to true and approvals_before_merge is set?

  Do we need a test for that? I didn't see one that specified this behavior, though I might have missed it.

  Should this go into a follow up issue?

Steps to reproduce

1. Project settings: Set No. of approvals required to 3
2. Project settings: Check the box Can override approvers and approvals required per merge request
3. Edit an MR: Set No. of approvals required to 1
4. Project settings: Uncheck the box Can override approvers and approvals required per merge request

I expect MR approvals required to be 3, but it remains at 1.