Update CHANGELOG.md for 12.9.0

[ci skip]

changelogs/unreleased/202271-migrate-fa-spinner-for-notifications_dropdown-js.yml

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-title: migrate fa spinner for notification_dropdown.js
-merge_request: 25141
-author: minghuan
-type: other

changelogs/unreleased/202274-migrate-fa-spinner-to-spinner.yml

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-title: New loading spinner for attachemnt uploads via discussion boxes
-merge_request: 25057
-author: Philip Jonas
-type: changed

changelogs/unreleased/202426-editor-lite-theme-preference.yml

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-title: In single-file editor set syntax highlighting theme according to user's preference
-merge_request: 26606
-author:
-type: changed

changelogs/unreleased/204723-nomethoderror-undefined-method-term_agreements-for-nil-nilclass.yml

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-title: Require a logged in user to accept or decline a term
-merge_request: 24771
-author:
-type: fixed

changelogs/unreleased/204774-quick-actions-executed-in-multiline-inline-code.yml

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-title: Fix quick actions executing in multiline inline code when placed on its own line
-merge_request: 24933
-author: Pavlo Dudchenko
-type: fixed

changelogs/unreleased/204801-add-instance-to-services.yml

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-title: Add instance column to services table
-merge_request: 25714
-author:
-type: other

changelogs/unreleased/204858-update-self-monitor-environments.yml

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-title: Add migration to create self monitoring project environment
-merge_request: 25289
-author:
-type: added

changelogs/unreleased/205184-change-omniauth-log-format-to-json.yml

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-title: Change OmniAuth log format to JSON
-merge_request: 25086
-author:
-type: other

changelogs/unreleased/205399-add-tooltip-to-file-tree-state.yml

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-title: Add tooltip to modification icon in the file tree
-merge_request: 27158
-author:
-type: other

changelogs/unreleased/205435.yml

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-title: Clean up conditional `col-` classes in `nav_dropdown_button.vue`
-merge_request: 25312
-author:
-type: other

changelogs/unreleased/205596-empty-state-for-code-review-analytics.yml

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-title: Empty state for Code Review Analytics
-merge_request: 25793
-author:
-type: added

changelogs/unreleased/206899-move-system-metrics-chart-group-to-the-top-of-the-default-dashbord.yml

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-title: Put System Metrics chart group first in default dashboard
-merge_request: 26355
-author:
-type: other

changelogs/unreleased/207087-blocked-status-issue.yml

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-title: Don't show issue as blocked on the issue board if blocking issue is closed
-merge_request: 25817
-author:
-type: fixed

changelogs/unreleased/207126-more-descriptive-error-messages-in-migration-helpers.yml

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-title: Improve error messages of failed migrations
-merge_request: 25457
-author:
-type: changed

changelogs/unreleased/207181-status-page-settings-backend.yml

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-title: Create table & setup operations endpoint for Status Page Settings
-merge_request: 25863
-author:
-type: added

changelogs/unreleased/207203-forward-deployment-ui.yml

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-title: Added Drop older active deployments project setting
-merge_request: 25520
-author:
-type: added

changelogs/unreleased/207216-lfs-batch-upload-fix.yml

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-title: Mark existing LFS object for upload for forks
-merge_request: 26344
-author:
-type: fixed

changelogs/unreleased/207223-fix-self-monitoring-project.yml

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-title: Fix self monitoring project link
-merge_request: 25516
-author:
-type: fixed

changelogs/unreleased/207237-snippet-edit-description-vue.yml

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-title: Added Blob Description Edit component in Vue
-merge_request: 26762
-author:
-type: added

changelogs/unreleased/207242-vsibility-level-vue.yml

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-title: Added Edit Visibility Vue compoenent for Snippet
-merge_request: 26799
-author:
-type: added