Update CHANGELOG.md for 12.9.0

[ci skip]

changelogs/unreleased/27072-name-regex-allow-bulk-api.yml

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-title: Add name_regex_keep param to container registry bulk delete API endpoint
-merge_request: 25484
-author:
-type: added

changelogs/unreleased/27144-gitlab-hosted-codesandbox.yml

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-title: Update Web IDE clientside preview bundler to use GitLab managed server
-merge_request: 21520
-author:
-type: changed

changelogs/unreleased/27227-widget-showing-changed-pages-for-visual-reviews.yml

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-title: Add changed pages dropdown to visual review modal
-merge_request:
-author:
-type: added

changelogs/unreleased/27300-add-filepath-redirect-url.yml

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-title: Add filepath redirect url
-merge_request: 25541
-author:
-type: added

changelogs/unreleased/27300-add-filepath-to-release-links-api.yml

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-title: Add filepath to release links API
-merge_request: 25533
-author:
-type: added

changelogs/unreleased/27300-enable-a-direct-link-to-a-release-and-release-assets-2.yml

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-title: Add filepath to ReleaseLink
-merge_request: 25512
-author:
-type: added

changelogs/unreleased/27300-expose-filepath-url-on-ui.yml

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-title: Expose assets filepath URL on UI
-merge_request: 25635
-author:
-type: added

changelogs/unreleased/27880-clearing-release-note-from-the-tags-page-deletes-release.yml

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-title: 27880 Make release notes optional and do not delete release when they are removed
-merge_request: 26231
-author: Pavlo Dudchenko
-type: changed

changelogs/unreleased/28085-index-options-tuning.yml

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-title: Optimize storage usage for newly created ES indices
-merge_request: 25992
-author:
-type: other

changelogs/unreleased/28560_cleanup_optimistic_locking_db.yml

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-title: Set all NULL `lock_version` values to 0 for issuables
-merge_request: 18418
-author:
-type: fixed

changelogs/unreleased/28627-adjust-commit-stats-over-limit-indication.yml

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-title: Add commits limit text at graphs page
-merge_request: 24990
-author:
-type: changed

changelogs/unreleased/28725-paginate-lfs-object-import.yml

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-title: Batch processing LFS objects downloads
-merge_request: 26434
-author:
-type: changed

changelogs/unreleased/31289-show-issue-summary-on-releases-page.yml

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-title: Cache milestone issue counters and make them independent of user permissions
-merge_request: 21554
-author:
-type: performance

changelogs/unreleased/32046-differentiate-between-errors-failures-in-xunit-result.yml

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-title: Differentiate between errors and failures in xUnit result
-merge_request: 23476
-author:
-type: changed

changelogs/unreleased/32882-render-special-references-for-releases.yml

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-title: Render special references for releases
-merge_request: 26554
-author:
-type: fixed

changelogs/unreleased/33641-fix_smartcard_param_check_in_user_build.yml

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-title: Fix user registration when smartcard authentication is enabled
-merge_request: 26800
-author:
-type: fixed

changelogs/unreleased/34086-es-bulk-incremental-index-updates.yml

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-title: 'Add a bulk processor for elasticsearch incremental updates'
-merge_request: 24298
-author:
-type: added

changelogs/unreleased/34420-optimize-pagination-on-explore-snippets.yml

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-title: Showing only "Next" button for snippet explore page.
-merge_request: 25404
-author:
-type: changed

changelogs/unreleased/34525-update-custom-dashboard.yml

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-title: Update file content of an existing custom dashboard
-merge_request: 25024
-author:
-type: added

changelogs/unreleased/35475-add-prometheus-ci-vars.yml

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-title: Support more query variables in custom dashboards per project
-merge_request: 25732
-author:
-type: added