danger: Make sure a feature flag removal comes with a changelog [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

This makes sure feature flag removals are visible by GitLab users.

Screenshots (strongly suggested)

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

Related to #301086 (closed).