Update CHANGELOG.md for 12.9.0

[ci skip]

changelogs/unreleased/196688-replace-underscore-with-lodash-for-app-assets-javascripts-deploy_k.yml

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-title: Replace underscore with lodash for ./app/assets/javascripts/deploy_keys
-merge_request: 24965
-author: Jacopo Beschi @jacopo-beschi
-type: changed

changelogs/unreleased/196718-remove-filter-epic-counts.yml

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-title: Remove visibility check from epic descendant counts
-merge_request: 25975
-author:
-type: changed

changelogs/unreleased/196832-drop-feature-toggle.yml

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-title: Optimize project representation in large imports
-merge_request: !22598
-author:
-type: performance

changelogs/unreleased/196883-repository-link-grpc-graceful-failure.yml

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-title: Ensure RepositoryLinkFilter handles Gitaly failures gracefully
-merge_request: 26531
-author:
-type: performance

changelogs/unreleased/197227-milestone-tab-async.yml

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-title: Search issues in GraphQL API by milestone title and assignees
-merge_request: 25794
-author:
-type: added

changelogs/unreleased/197480-remove-package-file_type.yml

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-title: Remove unused file_type column from packages_package_files
-merge_request: 26527
-author:
-type: changed

changelogs/unreleased/197790-hide-private-commit-emails-in-notification-settings.yml

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-title: Hide the private commit email in Notification email list
-merge_request: 25099
-author: briankabiro
-type: changed

changelogs/unreleased/197918-add-package-type-param-to-group-packages-api.yml

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-title: Add package_type as a filter option to the group packages list API endpoint
-merge_request: 26833
-author:
-type: added

changelogs/unreleased/197918-add-package-type-to-project-packages-api.yml

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-title: Add package_type as a filter option to the packages list API endpoint
-merge_request: 25816
-author:
-type: added

changelogs/unreleased/197960-package-detail-activity.yml

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-title: Adds new activity panel to package details page
-merge_request: 25534
-author:
-type: added

changelogs/unreleased/198050-frontend-pagination-in-log-explorer.yml

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-title: More logs entries are loaded when logs are scrolled to the top
-merge_request: 26254
-author:
-type: added

changelogs/unreleased/198052-in-the-jobs-page-the-allowed-to-fail-badge-looks-like-an-error.yml

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-title: Hides the "Allowed to fail" tag on jobs that are successful
-merge_request: 25458
-author:
-type: changed

changelogs/unreleased/198325-migrate-design-mentions-to-db-table.yml

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-title: Migrate mentions for design notes to design_user_mentions DB table
-merge_request: 23704
-author:
-type: changed

changelogs/unreleased/198326-migrate-commit-notes-mentions-to-db-table.yml

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-title: Migrate mentions for commit notes to commit_user_mentions DB table
-merge_request: 23859
-author:
-type: changed

changelogs/unreleased/198338-migrate-mr-mentions-to-db-table.yml

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-title: Migrate mentions for merge requests to DB table
-merge_request: 25826
-author:
-type: changed

changelogs/unreleased/198391-add-user-plan-and-trial-status-to-api.yml

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-title: Expose `plan` and `trial` to `/users/:id` endpoint
-merge_request: 25151
-author:
-type: added

changelogs/unreleased/198604-monaco-snippets.yml

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-title: Replaced ACE with Monaco editor for Snippets
-merge_request: 25465
-author:
-type: added

changelogs/unreleased/19880-sort-closed-issues-by-recently-closed.yml

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-title: Sort closed issues on issue boards using time of closing
-merge_request: 23442
-author: briankabiro
-type: changed

changelogs/unreleased/199197-make-has-been-reverted-check-cheaper.yml

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-title: Improve performance of the "has this commit been reverted?" check
-merge_request: 26784
-author:
-type: performance

changelogs/unreleased/199220-hide-snippet-blob-search.yml

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-title: Remove and deprecate snippet content search
-merge_request: 26359
-author:
-type: removed