Move Admin Area Operations menu to top-level

What does this MR do?

In !21222 (merged) (originally proposed by #35591 (closed)), we added a new setting to the Admin Area for configuring an instance-level serverless domain. This setting was added under Settings > Operations.

This MR proposes that this setting be moved under a new top-level Operations menu. There are a few reasons for this:

• All the other settings under Settings are fields on the ApplicationSetting model; Serverless domains do not follow this pattern.
• Having a top-level Operations menu would make the structure consistent with that of Projects and Groups, which both have serverless settings under Operations > Serverless. For the same reason, we might also consider moving Kubernetes under this new Operations menu.
• Serverless domains aren't really a setting, so much as they are resources that can be created and destroyed, similar to Kubernetes clusters.

Screenshots

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After:

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