Restrict group level approval settings to top level group

What does this MR do?

Restrict group MR approval settings to root group

This commit restricts the permission to administer group merge request approval settings to root group. Sub groups (i.e. groups which have parent groups) will not be able to access this feature.

Recursive settings resolution of deeply nested group hierarchy is computation-extensive. For the MVP, we would like to mitigate this risk and re-evaluate this feature on nested group after collecting more user feedback.

Testing

1. Enable the feature flag in rails console: Feature.enable(:group_merge_request_approval_settings_feature_flag)
2. Navigate to the root group settings page Group > Settings > General
3. Confirm that the Merge Request Approval section is visible
4. Navigate to the subgroup group settings page Group > Settings > General
5. Confirm that the Merge Request Approval section is not visible

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because this feature is behind a feature flag which is disabled by default.
• 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

Related to #324037 (closed)