Update CHANGELOG.md for 12.9.0

[ci skip]

changelogs/unreleased/fix-prevent-user-theme-color-api-overwrite.yml

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-title: Prevent default overwrite for theme and color ID in user API
-merge_request: 26792
-author: Fabio Huser
-type: fixed

changelogs/unreleased/fixes-caret-position-after-pasting-an-image-15011.yml

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-title: Fixes caret position after pasting an image 15011
-merge_request: 21382
-author: Carolina Carvalhosa
-type: fixed

changelogs/unreleased/fj-195517-single-blob-snippet-view-render.yml

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-title: Render single snippet blob in repository
-merge_request: 23848
-author:
-type: added

changelogs/unreleased/fj-205646-fix-project-moved-message.yml

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-title: Fix project moved message after git operation
-merge_request: 27341
-author:
-type: fixed

changelogs/unreleased/fj-207803-fix-project-snippet-policy-bug.yml

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-title: Fix bug deleting internal project snippets by project maintainer
-merge_request: 25792
-author:
-type: fixed

changelogs/unreleased/fj-208693-fix-bug-creating-snippet.yml

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-title: Fix bug committing snippet content when creating the snippet
-merge_request: 26287
-author:
-type: fixed

changelogs/unreleased/fj-39176-create-project-snippet-repository.yml

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-title: Update git workflows and routes to allow snippets
-merge_request: 21739
-author:
-type: added

changelogs/unreleased/fj-39201-import-export-project-snippets.yml

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-title: Import/Export snippet repositories
-merge_request: 24150
-author:
-type: added

changelogs/unreleased/fj-39265-create-snippet-repository-content.yml

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-title: Commit file when snippet is created
-merge_request: 23953
-author:
-type: added

changelogs/unreleased/fj-39265-update-snippet-repository-content.yml

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-title: Update files when snippet is updated
-merge_request: 23993
-author:
-type: changed

changelogs/unreleased/fj-39515-delete-snippet-repositories.yml

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-title: Add/update services to delete snippets repositories
-merge_request: 22672
-author:
-type: added

changelogs/unreleased/fj-allow-create-snippet-default-branch.yml

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-title: Allow creating default branch in snippet repositories
-merge_request: 26294
-author:
-type: fixed

changelogs/unreleased/fj-fix-bug-hook-env.yml

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-title: Fix bug setting hook env with personal snippets
-merge_request: 27235
-author:
-type: fixed

changelogs/unreleased/fj-fix-git-error-message-update-snippet.yml

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-title: Show git error message updating snippet
-merge_request: 26570
-author:
-type: fixed

changelogs/unreleased/fj-fix-internal-api-return-code.yml

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-title: Change back internal api return code
-merge_request: 26063
-author:
-type: fixed

changelogs/unreleased/fj-fix-snippet-update-error-message.yml

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-title: Fix bug displaying snippet update error
-merge_request: 27082
-author:
-type: fixed

changelogs/unreleased/fj-fix-snippet-url-to-repo.yml

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-title: Fix remove special chars from snippet url_to_repo
-merge_request: 27390
-author:
-type: fixed

changelogs/unreleased/fj-remove-repository-storage-column-from-snippets.yml

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-title: Remove repository_storage column from snippets
-merge_request: 25699
-author:
-type: other

changelogs/unreleased/fj-rename-unauthorized-error-to-forbidden-error.yml

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-title: Align git returned error codes
-merge_request: 25936
-author:
-type: changed

changelogs/unreleased/fj-update-snippet-from-git-action.yml

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-title: Sync snippet after Git action
-merge_request: 26565
-author:
-type: changed