Update CHANGELOG.md for 12.9.0

[ci skip]

changelogs/unreleased/feat-broadcast-message-dimsiss.yml

----
-title: Add user dismiss option to broadcast messages
-merge_request: 20665
-author: Fabio Huser
-type: added

changelogs/unreleased/feat-mr-diff-coverage-visualisation.yml

----
-title: Add Cobertura XML coverage visualization to merge request diff view
-merge_request: 21791
-author: Fabio Huser
-type: added

changelogs/unreleased/feat-x509-crl.yml

----
-title: Add functionality to revoke a X509Certificate and update related X509CommitSignatures
-merge_request: 24889
-author: Roger Meier
-type: added

changelogs/unreleased/feature-199458-track-jump-to-next-unresolved-thread.yml

----
-title: Added tracking to merge request jump to next thread buttons
-merge_request: 26319
-author: Martin Hobert
-type: added

changelogs/unreleased/feature-enable-split-diffs-by-default.yml

----
-title: Diffs load each view style separately, on demand
-merge_request: 24821
-author:
-type: performance

changelogs/unreleased/feature-image-diff-size.yml

----
-title: All image diffs (except for renamed files) show the image file size in the
-  diff
-merge_request: 25734
-author:
-type: added

changelogs/unreleased/file-path-validator.yml

----
-title: Add custom validator for validating file path
-merge_request: 24223
-author: Rajendra Kadam
-type: added

changelogs/unreleased/filter-sentry-error-list.yml

----
-title: Filter sentry error list by status (unresolved/ignored/resolved)
-merge_request: 26205
-author:
-type: added

changelogs/unreleased/find-commits-by-author.yml

----
-title: Filter commits by author
-merge_request: 25597
-author:
-type: added

changelogs/unreleased/fix-api.yml

----
-title: Support finding namespace by ID or path on fork API
-merge_request: 20603
-author: leoleoasd
-type: fixed

changelogs/unreleased/fix-dependency-proxy-link.yml

----
-title: Add link to dependency proxy docs on the dependency proxy page
-merge_request: 26092
-author:
-type: changed

changelogs/unreleased/fix-deployment-namespace-resolution.yml

----
-title: Fix Kubernetes namespace resolution for new DeploymentCluster records
-merge_request: 25853
-author:
-type: fixed

changelogs/unreleased/fix-deployments-pagination.yml

----
-title: Add API pagination for deployed merge requests
-merge_request: 25733
-author:
-type: performance

changelogs/unreleased/fix-duplicate-labels-when-moving-projects.yml

----
-title: Fix duplicate labels when moving projects within the same ancestor group
-merge_request: 27261
-author:
-type: fixed

changelogs/unreleased/fix-export-state-logic.yml

----
-title: Fix logic to determine project export state and add regeneration_in_progress state
-merge_request: 23664
-author:
-type: fixed

changelogs/unreleased/fix-merge-to-ref-service-raise-command-error.yml

----
-title: Fix MergeToRefService raises Gitlab::Git::CommandError
-merge_request: 26465
-author:
-type: fixed

changelogs/unreleased/fix-mermaid-flow-chart-width.yml

----
-title: Fix Mermaid flowchart width
-merge_request: 26848
-author: julien MILLAU
-type: fixed

changelogs/unreleased/fix-pipeline-creation-race-conditions.yml

----
-title: Drop bridge if downstream pipeline has errors
-merge_request: 25706
-author:
-type: fixed

changelogs/unreleased/fix-pipeline-details-invalid-buttons.yml

----
-title: Fix pipeline details page initialisation on invalid pipeline
-merge_request: 25302
-author: Fabio Huser
-type: fixed

changelogs/unreleased/fix-pipeline-tooltip.yml

----
-title: Change tooltip text for pipeline on last commit widget
-merge_request: 26315
-author:
-type: other