Restore changelog requirements

What does this MR do?

Restore changelog requirements in our documentation

The new changelog workflow has been rolled out gitlab-com/gl-infra/delivery#1564 (closed), while we no longer require changelog entries in changelogs/unreleased we still require specific MRs to provide changelog messages using the new workflow.

This merge request restores the MUSTs changelog rules

Does this MR meet the acceptance criteria?

Conformity

• [-] I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• [-] I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• [-] I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.