Update MRs authors responsibility

What does this MR do?

Make MRs authors responsible for setting milestones and labels on their MRs.

Inspired by frontend review guidelines: https://docs.gitlab.com/ee/development/fe_guide/development_process.html#merge-request-review

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