Remove approvals_before_merge/approvals_required fields from public API (breaking change)

The approvals_before_merge field has been made redundant within:

• Issue: #11132 (closed).
• Merge request: !16121 (merged)

Within this issue, we should remove approvals_before_merge/approvals_required fields from public API or Projects and Merge Requests. It's a breaking change and must be done within a major release %15.0

It includes:

• approvals_before_merge API field
• Updating documentation: https://docs.gitlab.com/ee/api/merge_request_approvals.html
• Removing /approvals endpoint