Link to issue in CodeReuse/ActiveRecord message

What does this MR do?

This MR adds a link to https://gitlab.com/gitlab-org/gitlab-ce/issues/49653 within the CodeReuse/ActiveRecord Rubocop message.

This provides:

• context for developers to understand the reasoning behind the rule
• information about how to go about implementing a code solution

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