[skip ci] Fix changelog for 6.6.6
What does this MR do?
Fix the changelog entry for version 6.6.6.
What are the relevant issue numbers?
Does this MR meet the acceptance criteria?
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Changelog trailer 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 -
Conforms to the code review guidelines -
Conforms to the Go guidelines -
Security reports checked/validated by reviewer