Update CHANGELOG.md for 12.0.0

[ci skip]

changelogs/unreleased/37495.yml

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-title: Add documentation links for confidental and locked discussions
-merge_request: 29073
-author:
-type: changed

changelogs/unreleased/38561-border-radii.yml

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-title: Fix border radii on diff files and repo files
-merge_request:
-author:
-type: fixed

changelogs/unreleased/39304-broadcast-message-buttons.yml

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-title: Update broadcast message action icons
-merge_request: 28496
-author: Jarek Ostrowski @jareko
-type: fixed

changelogs/unreleased/45687-web-ide-empty-state.yml

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-title: Empty project state for Web IDE
-merge_request: 26556
-author:
-type: added

changelogs/unreleased/47846-position-is-off-when-visiting-files-with-anchors.yml

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-title: Resolve Position is off when visiting files with anchors
-merge_request: 28913
-author:
-type: fixed

changelogs/unreleased/49041-issue-board-input-height.yml

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-title: Reduce height of issue board input to align with buttons
-merge_request:
-author:
-type: other

changelogs/unreleased/49517-fix-notes-import-export.yml

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-title: Fix diff notes and discussion notes being exported as regular notes
-merge_request: 28401
-author:
-type: fixed

changelogs/unreleased/49915-fix-error-500-admin-projects-nil-storage.yml

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-title: Fix an error in projects admin when statistics are missing
-merge_request: 28355
-author:
-type: fixed

changelogs/unreleased/50106-hide-whitespace-changes.yml

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-title: Fix whitespace changes visibility when the related file was initially collapsed
-merge_request: 28950
-author: Ondřej Budai
-type: fixed

changelogs/unreleased/50850-kerrizor-extend-api-to-accept-start_project-option.yml

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-title: Add API support for committing changes to different projects in same fork network
-merge_request: 27915
-author:
-type: added

changelogs/unreleased/51022-added-extended-height-to-labels-dropdown.yml

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-title: "Added the `.extended-height` class to the labels-dropdown"
-merge_request: 28659
-author: Michel Engelen
-type: other

changelogs/unreleased/51636-task-list-api-pderichs.yml

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-title: Add task count and completed count to responses of Issue and MR
-merge_request: 28859
-author:
-type: added

changelogs/unreleased/51854-api-to-get-all-project-group-members-returns-duplicates.yml

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-title: Removes duplicated members from api/projects/:id/members/all
-merge_request: 24005
-author: Jacopo Beschi @jacopo-beschi
-type: changed

changelogs/unreleased/53134-multiple-extendes-for-a-job.yml

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-title: Add support for multiple job parents in GitLab CI YAML.
-merge_request: 26801
-author: Wolphin (Nikita)
-type: added

changelogs/unreleased/54140-non-ar-cache-commit-markdown.yml

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-title: Use Redis for CacheMarkDownField on non AR models
-merge_request: 29054
-author:
-type: performance

changelogs/unreleased/54169-flash-warning-rebrush.yml

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-title: "Rebrush of flash-warning according to the new design (brighter background and darker font)"
-merge_request: 28916
-author: Michel Engelen
-type: changed

changelogs/unreleased/55033-discussion-system-note-alignment.yml

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-title: Align system note within discussion with other notes
-merge_request:
-author:
-type: fixed

changelogs/unreleased/55125-mr-tab-scrolling.yml

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-title: Update merge request tabs so they no longer scroll
-merge_request:
-author:
-type: other

changelogs/unreleased/55253-activity-feed-ui-enhance-line-height.yml

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-title: Enhance line-height of Activity feed UI
-merge_request: 28856
-author: Jacopo Beschi @jacopo-beschi
-type: changed

changelogs/unreleased/55362-refresh-blank-service-account-token.yml

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-title: Refresh service_account_token for kubernetes_namespaces
-merge_request: 29657
-author:
-type: fixed