Update CHANGELOG.md for 12.9.0

[ci skip]

changelogs/unreleased/mo-change-capybara-screenshots-name.yml

----
-title: Change capybara screenshots files names taken on tests failures
-merge_request: 26788
-author:
-type: changed

changelogs/unreleased/mo-fix-capybara-screenshots-rails.yml

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-title: Fix capybara screenshots path name for rails configuration
-merge_request: 27002
-author:
-type: fixed

changelogs/unreleased/mo-use-new-code-quality-image.yml

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-title: Use new codequality docker image from ci-cd group
-merge_request: 27098
-author:
-type: other

changelogs/unreleased/move-storage-shards.yml

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-title: "Backport API support to move between repository storages/shards"
-merge_request: 18721
-author: Ben Bodenmiller
-type: added

changelogs/unreleased/mwaw-197871-improve-duplicated-dashboard-error-messages.yml

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-title: Fix error messages for dashboard clonning process.
-merge_request: 26290
-author:
-type: fixed

changelogs/unreleased/mwaw-activate_shared_services_on_project_creation.yml

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-title: Activate Prometheus integration service for newly created project if this project
-  has access to shared Prometheus application.
-merge_request: 24676
-author:
-type: fixed

changelogs/unreleased/mwaw-remove_logs_path_for_not_authorised_users.yml

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-title: Remove unreachable link from embded dashboard context menu
-merge_request: 25892
-author:
-type: fixed

changelogs/unreleased/nfriend-create-release-through-ui.yml

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-title: Add "New release" button to Releases page
-merge_request: 24516
-author:
-type: added

changelogs/unreleased/nfriend-enable-issues-summary.yml

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-title: Add issue summary to Release blocks on the Releases page
-merge_request: 27032
-author:
-type: added

changelogs/unreleased/nfriend-enable-release-show-page-feature-flag.yml

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-title: Add dedicated Release page for viewing a single Release
-merge_request: 26502
-author:
-type: added

changelogs/unreleased/nicolasdular-email-restriction-regex.yml

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-title: Add restrictions for signup email addresses
-merge_request: 25122
-author:
-type: added

changelogs/unreleased/open-project-integration-2.yml

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-title: Add migration for creating open_project_tracker_data table
-merge_request: 26966
-author:
-type: other

changelogs/unreleased/osw-move-sidekiq-cluster-to-core.yml

----
-title: Move sidekiq-cluster script to Core
-merge_request: 26703
-author:
-type: other

changelogs/unreleased/osw-support-opt-in-cluster-in-bg-jobs-script.yml

----
-title: Support sidekiq-cluster supervision through bin/background_jobs
-merge_request: 27042
-author:
-type: added

changelogs/unreleased/pages-1-17.yml

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-title: Upgrade Pages to 1.17.0
-merge_request: 26478
-author:
-type: added

changelogs/unreleased/persist-expanded-environment-name-in-build-metadata.yml

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-title: Persist expanded environment name in ci build metadata
-merge_request: 22374
-author:
-type: performance

changelogs/unreleased/pokstad1-remove-branch-user-squash.yml

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-title: Upgrade Gitaly gem and fix UserSquash RPC usage
-merge_request: 27372
-author:
-type: other

changelogs/unreleased/public-api-for-merge-trains.yml

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-title: Allow users to get Merge Trains entries via Public API
-merge_request: 25229
-author:
-type: added

changelogs/unreleased/rc-whitelist_ports.yml

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-title: Add ability to whitelist ports
-merge_request: 27025
-author:
-type: added

changelogs/unreleased/refactor-bypass-session-admin-mode.yml

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-title: Sessionless and API endpoints bypass session for admin mode
-merge_request: 25056
-author: Diego Louzán
-type: changed