Show success message on MR approval settings save

What does this MR do?

This MR adds a flash message on top of the page when MR approval settings have been updated successfully.

This feature is behind a feature flag: group_merge_request_approval_settings_feature_flag

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry - Behind a 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 #285458 (closed)