Add Prevent users from modifying MR approval rules to group level

What does this MR do?

This MR adds a Prevent users from modifying MR approval rules setting to the group level Merge request approvals settings as described in #325989 (closed).

Setup & Testing

1. Enable the feature flag echo "Feature.enable(:group_merge_request_approval_settings_feature_flag)" | rails c
2. View the Merge request approvals settings at [GDK_HOST]/groups/[GROUP]/-/edit
3. Toggle the checkbox and refresh the page to see if the setting persists

Screenshots (strongly suggested)

Before	After

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 it's behind a development feature flag.
• [-] 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 #325989 (closed)