Update CHANGELOG.md for 12.9.0

[ci skip]

changelogs/unreleased/create-approval-todos-on-update.yml

----
-title: Create approval todos on update
-merge_request: 26077
-author:
-type: fixed

changelogs/unreleased/create-puma-rugged-suboptimal-config-checker.yml

----
-title: 'Show notices in Admin area when detected any of these cases: Puma, multi-threaded
-  Puma, multi-threaded Puma + Rugged'
-merge_request: 21403
-author:
-type: added

changelogs/unreleased/create-remote-mirrors-docs.yml

----
-title: Add documentation for create remote mirrors API
-merge_request: 26012
-author: Rajendra Kadam
-type: added

changelogs/unreleased/create_single_temporary_index_for_notes_mentions.yml

----
-title: Replace several temporary indexes with a single one to save time when running mentions migration
-merge_request:
-author:
-type: performance

changelogs/unreleased/create_users_statistics.yml

----
-title: Introduce db table to store users statistics
-merge_request: 26261
-author:
-type: added

changelogs/unreleased/creator-pairing-fix-alert.yml

----
-title: Fixed SSH warning style
-merge_request: 26992
-author:
-type: other

changelogs/unreleased/creator-pairing-group-list-padding.yml

----
-title: Added a padding-right to items in subgroup list
-merge_request: 26791
-author:
-type: fixed

changelogs/unreleased/dblessing-add-scim-identities.yml

----
-title: Create scim_identities table in preparation for newer SCIM features in the
-  future
-merge_request: 26124
-author:
-type: added

changelogs/unreleased/dennis-update-ios-swift-project-template-logo.yml

----
-title: Update iOS (Swift) project template logo
-merge_request: 25049
-author:
-type: changed

changelogs/unreleased/deploy-mr-once-take-2.yml

----
-title: Don't track MR deployment multiple times
-merge_request: 25537
-author:
-type: fixed

changelogs/unreleased/descriptive_pages_error.yml

----
-title: Use clearer error message for pages deploy job when the SHA is outdated
-merge_request: 25659
-author:
-type: other

changelogs/unreleased/dmishunov-rich-viewers.yml

----
-title: Special handling for the rich viewer on specific file types
-merge_request: 26260
-author:
-type: changed

changelogs/unreleased/do-not-parse-undefined-severity-confidence.yml

----
-title: Do not parse undefined severity and confidence from reports
-merge_request: 25884
-author:
-type: other

changelogs/unreleased/dotenv-report-artifact.yml

----
-title: Support DotEnv Variables through report type artifact
-merge_request: 26247
-author:
-type: added

changelogs/unreleased/drop_forked_project_links_table.yml

----
-title: Drop forked_project_links table
-merge_request: 20771
-author: Lee Tickett
-type: other

changelogs/unreleased/drop_old_state_column_from_issues.yml

----
-title: Remove state column from issues and merge_requests
-merge_request: 25561
-author:
-type: deprecated

changelogs/unreleased/dz-scope-issue-route-default.yml

----
-title: Move issues routes under /-/ scope
-merge_request: 24791
-author:
-type: changed

changelogs/unreleased/ee-insert-all-for-load-balancing.yml

----
-title: Support Rails 6 `insert_all!`
-merge_request: 26595
-author:
-type: fixed

changelogs/unreleased/enable-customizable-cycle-analytics.yml

----
-title: Enable customizable_cycle_analytics feature flag by default
-merge_request: 27418
-author:
-type: changed

changelogs/unreleased/feat-2fa-for-admin-mode.yml

----
-title: Add 2FA support to admin mode feature
-merge_request: 22281
-author: Diego Louzán
-type: added