chore: Add maintenance-mode warnings to issue and command modules

What does this MR do?

These modules are no longer maintained and can lead to unexpected bugs. They should no longer be used in favor of their separately maintained equivalents.

What are the relevant issue numbers?

See gitlab-org/gitlab#211819 (closed) for more details

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated for GitLab EE, if necessary
• Documentation created/updated for this project, if necessary
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Job definition updated, if necessary
  • Auto-DevOps template
  • Job definition example
  • CI Templates
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer