Update CHANGELOG.md for 12.9.0

[ci skip]

changelogs/unreleased/21811-instance-deploy-tokens.yml

----
-title: Add deploy tokens instance API endpoint
-merge_request: 25066
-author:
-type: added

changelogs/unreleased/21811-project-create-deploy-tokens.yml

----
-title: Add api endpoint to create deploy tokens
-merge_request: 25270
-author:
-type: added

changelogs/unreleased/21811-project-delete-deploy-token.yml

----
-title: Add API endpoint for deleting project deploy tokens
-merge_request: 25220
-author:
-type: added

changelogs/unreleased/21811-project-list-deploy-tokens.yml

----
-title: Add endpoint for listing all deploy tokens for a project
-merge_request: 25186
-author:
-type: added

changelogs/unreleased/22103-make-code-tags-consistent-in-discussions.yml

----
-title: Deemphasized styles for inline code blocks
-merge_request:
-author:
-type: changed

changelogs/unreleased/24072-user-profile-add-job-title.yml

----
-title: Add "Job Title" field in user settings and display on profile
-merge_request: 25155
-author:
-type: added

changelogs/unreleased/24083-tableflip-quick-action-is-interpreted-even-if-inside-code-block.yml

----
-title: 'Fix: tableflip quick action is interpreted even if inside code block'
-merge_request:
-author: Pavlo Dudchenko
-type: fixed

changelogs/unreleased/24309-notifications-for-when-pipelines-are-fixed.yml

----
-title: Notifications for when pipelines are fixed
-merge_request: 16951
-author: Jacopo Beschi @jacopo-beschi
-type: added

changelogs/unreleased/25095-remove-gitlab-shell-indirection-for-authorized-keys.yml

----
-title: Move authorized_keys operations into their own Sidekiq queue
-merge_request: 26913
-author:
-type: changed

changelogs/unreleased/25283-add-masked-param-group-vars-api.yml

----
-title: "Allow to create masked variable from group variables API"
-merge_request: 25283
-author: Emmanuel CARRE
-type: added

changelogs/unreleased/25334-update-rouge.yml

----
-title: Update rouge to v3.16.0
-merge_request: 25334
-author: Konrad Borowski
-type: other

changelogs/unreleased/25351-add-buttons.yml

----
-title: Add deploy and re-deploy buttons to deployments
-merge_request: 25427
-author:
-type: added

changelogs/unreleased/25550-there-is-a-drag-and-drop-bug-in-boards.yml

----
-title: Resolves the disappearance of a ticket when it was moved from the closed list.
-merge_request:
-author: Gwen_
-type: fixed

changelogs/unreleased/25744-optional-custom-icon-in-omniauth-login-label.yml

----
-title: Optional custom icon in the OmniAuth login labels
-merge_request: 25744
-author: Tobias Wawryniuk, Luca Leonardo Scorcia
-type: added

changelogs/unreleased/25838-include-full-upload-url-in-api-response.yml

----
-title: Include full path to an upload in api response
-merge_request: 23500
-author: briankabiro
-type: other

changelogs/unreleased/25995-default-relative-links-to-blobs.yml

----
-title: Default to generating blob links for missing paths
-merge_request: 26817
-author:
-type: fixed

changelogs/unreleased/26111-fix-github-gist-links.yml

----
-title: Fix submodule links to gist.github.com
-merge_request: 27346
-author:
-type: fixed

changelogs/unreleased/26113-file-type-issue.yml

----
-title: Fix issues with non-ASCII plain text files being incorrectly uploaded as binary
-  in the Web IDE
-merge_request: 26360
-author:
-type: fixed

changelogs/unreleased/26556-create-merge-request-button-extends-past-edge-on-mobile.yml

----
-title: Prevent "Select project to create merge request" button from overflowing out
-  of the viewport on mobile
-merge_request: 25195
-author:
-type: fixed

changelogs/unreleased/26712-Update-GitLab-codeclimate-to-head.yml

----
-title: Update GitLab's codeclimate to 0.85.9
-merge_request: 26712
-author: Eddie Stubbington
-type: other