Fix: undefined background migration classes for EE-CE downgrade

What does this MR do?

This MR moves EE background migrations to CE scope and extends them on EE side so it won't fail when we update EE to CE and there are still some jobs spawned. If we agree on this change we probably also need to update https://docs.gitlab.com/ee/development/ee_features.html and https://docs.gitlab.com/ee/development/background_migrations.html. Also, a special Rubocop checker would be nice to have for this.

When I moved the code, the migrations stopped working because of EE namespace so I needed to adjust the code a bit.

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

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

Closes #30229 (closed)