Add missing changelog for v2.6.0
What does this MR do?
!18 (merged) was missing a CHANGELOG entry. This MR adds the entry so we can release a new version.
What are the relevant issue numbers?
Does this MR meet the acceptance criteria?
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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