Update CHANGELOG.md for 12.9.0

[ci skip]

changelogs/unreleased/199220-snippet-index-desc.yml

----
-title: Add trigram index on snippet description
-merge_request: 26341
-author:
-type: performance

changelogs/unreleased/199220-snippet-search.yml

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-title: Include snippet description as part of snippet title search (basic search).
-merge_request: 25961
-author:
-type: added

changelogs/unreleased/199370-move-deploy_keys-to-cicd.yml

----
-title: Moved Deploy Keys from Repository to CI/CD settings.
-merge_request: 25444
-author:
-type: changed

changelogs/unreleased/199400-send-email-notifications-for-generic-alerts.yml

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-title: Send alert emails for generic incident alerts
-merge_request: 24414
-author:
-type: added

changelogs/unreleased/199422-maximum-size-for-gitlab-pages-says-to-set-it-to-0-for-unlimited-bu.yml

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-title: Allow 0 to be set for pages maximum size per project/group to indicate unlimited
-  size
-merge_request: 25677
-author:
-type: fixed

changelogs/unreleased/199438-fix-logs-encoding.yml

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-title: Fix 500 error caused by Kubernetes logs not being encoded in UTF-8
-merge_request: 25999
-author:
-type: fixed

changelogs/unreleased/199442-explore-projects.yml

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-title: Handle Gitaly failure when fetching license
-merge_request: 24310
-author:
-type: fixed

changelogs/unreleased/199790-approval-settings-target-branch-api.yml

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-title: Filter rules by target_branch in approval_settings
-merge_request: 26439
-author:
-type: added

changelogs/unreleased/199908-use-only-the-first-line-of-the-commit-message-on-chat-service-noti.yml

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-title: Use only the first line of the commit message on chat service notification
-merge_request: 25224
-author: Takuya Noguchi
-type: changed

changelogs/unreleased/199998-container-expiration-policy-settings-hide-form-on-api-error-2.yml

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-title: Container expiration policy settings hide form on API error
-merge_request: 26303
-author:
-type: fixed

changelogs/unreleased/200107-avatar-content-type-does-not-match-file-extension.yml

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-title: Replace avatar and favicon upload type consistency validation with content whitelist validation
-merge_request: 25401
-author:
-type: changed

changelogs/unreleased/20083-conflict-between-project-s-permission-settings-description-and-actu.yml

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-title: Update project's permission settings description to reflect actual permissions
-merge_request: 25523
-author:
-type: other

changelogs/unreleased/201427-issue-board-due-date-picker-unable-to-select-custom-year-or-month.yml

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-title: Added the multiSelect option to stop event propagation when clicking on the
-  dropdown
-merge_request: 24611
-author: Gwen_
-type: fixed

changelogs/unreleased/201771.yml

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-title: Replace content_viewer_spec setTimeouts with semantic actions / events
-merge_request:
-author: Oregand
-type: other

changelogs/unreleased/201931-white-syntax-highlighting-theme-for-web-ide.yml

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-title: Fix White syntax highlighting theme in Monaco to closely match the Pygments theme.
-merge_request: 25966
-author:
-type: fixed

changelogs/unreleased/201999-define-formatter-y-axis.yml

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-title: Add properties to the dashboard definition to customize y-axis format
-merge_request: 25785
-author:
-type: added

changelogs/unreleased/201999-formatter-column-chart.yml

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-title: Use y-axis format configuration in column charts
-merge_request: 26356
-author:
-type: changed

changelogs/unreleased/202008-disable-drag-from-epic-tree-dropdown-button.yml

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-title: Disable draggable behavior on the epic tree chevron (collapse/expand) button
-merge_request: 25729
-author:
-type: changed

changelogs/unreleased/202094-enable-ff-by-default.yml

----
-title: Store first commit's authored_date for value stream calculation on merge
-merge_request: 26885
-author:
-type: changed

changelogs/unreleased/202233-migrating-fa-spinner-whithin-top-level-projects.yml

----
-title: Update loader for various project views
-merge_request: 25755
-author: Phellipe K Ribeiro
-type: other