Add ApprovalMRRule fallback for legacy code_owner rules

What does this MR do?

With the merger of https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/13036 a bug was introduced. Due to our Canary deployment process code can be pushed up that must not rely on post_migrate migrations to have ran. These only occur once a release has fully been rolled out, so any application logic on the canary instance must be self-contained to handle regressions.

Fixes https://gitlab.com/gitlab-org/gitlab-ee/issues/12035

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance 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