Resolve "Create a top-level Merge Request section under Settings"

What does this MR do?

This MR adds a new "Merge requests" subitem in "Setting" in the left sidebar. Merge request settings currently live under "Settings > General > Merge requests". These were copied over to the new settings menu. This new functionality is behind the :sidebar_refactor feature flag. When enabled, merge request settings will be available at "Settings > Merge requests", and won't show up at "Settings > General > Merge requests".

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 _____.
• 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 #119451 (closed)