Allow only admins to modify MR approval settings at the project-level

What does this MR do?

This MR removes the relationship between instance-level and project-level MR approval rules settings. This is a step to allow us moving towards a more holistic approach for scoping these rules on selected projects (i.e. with compliance labels).

🎈 Breaking changes: 🎈 Instance-level MR approvals settings no longer affect project-level ones.

We have decided to hide instance-level MR approvals settings under a feature flag: admin_merge_request_approvals_settings

Screenshots

Project (Admin)	Project (Maintainer)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team

Relates to #207250 (closed)