Fix module namespacing for mentionable concerns

What does this MR do?

Move background migration model concerns under the Concerns:: namespace.

When we switch to Zeitwerk, the module namespace must be consistent with the filesystem location. The reason we don't need the Concerns:: module in app/models/concerns is because that folder is part of autoload_paths so it's treated as a top-level folder.

This background migration folder isn't so we need to add Concerns::

This was extracted from !20521 (merged) for easier review

Screenshots

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