Update CHANGELOG.md for 9.1.0

[ci skip]

changelogs/unreleased/24421-personal-milestone-count-badges.yml

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-title: Add dashboard and group milestones count badges
-merge_request: 9836
-author: Alex Braha Stoll

changelogs/unreleased/24501-new-file-existing-branch.yml

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-title: New file from interface on existing branch
-merge_request: 8427
-author: Jacopo Beschi @jacopo-beschi

changelogs/unreleased/24784-system-notes-meta-data.yml

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-title: Add metadata to system notes
-merge_request: 9964
-author:

changelogs/unreleased/24861-stringify-group-member-details.yml

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-title: Hide form inputs for group member without editing rights
-merge_request: 7816
-author:

changelogs/unreleased/25188-polyfill-es-symbol.yml

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-title: Add ECMAScript polyfills for Symbol and Array.find
-merge_request: 10120
-author:

changelogs/unreleased/25332-make-file-templates-easy-to-use-and-discover.yml

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-title: Remove no-new annotation from file_template_mediator.js.
-merge_request: !9782
-author:

changelogs/unreleased/25515-delegate-single-discussion-to-new-issue.yml

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-title: Create a new issue for a single discussion in a Merge Request
-merge_request: 8266
-author: Bob Van Landuyt

changelogs/unreleased/25556-prevent-users-from-disconnecting-gitlab-account-from-cas.yml

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-title: Prevent users from disconnecting GitLab account from CAS
-merge_request: 10282
-author:

changelogs/unreleased/26188-tag-creation-404-for-guests.yml

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-title: Don't show links to tag a commit for users that are not permitted
-merge_request: 8407
-author:

changelogs/unreleased/26202-change-dropdown-style-slightly.yml

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-title: Changed dropdown style slightly
-merge_request:
-author:

changelogs/unreleased/26236-monospace-gfm.yml

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-title: Change gfm textarea to use monospace font
-merge_request:
-author:

changelogs/unreleased/26470-branch-names-with-reference-prefixes-results-in-buggy-branches.yml

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-title: Strip reference prefixes on branch creation
-merge_request: 8498
-author: Matthieu Tardy

changelogs/unreleased/27114-add-undo-mark-all-as-done-to-todos.yml

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-title: Add Undo mark all as done to Todos
-merge_request: 9890
-author: Jacopo Beschi @jacopo-beschi

changelogs/unreleased/27114-add-undo-to-todos-in-the-done-tab.yml

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-title: Add Undo to Todos in the Done tab
-merge_request: 8782
-author: Jacopo Beschi @jacopo-beschi

changelogs/unreleased/27174-filter-filters.yml

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-title: Prevent filtering issues by multiple Milestones or Authors
-merge_request:
-author:

changelogs/unreleased/27262-issue-recent-searches.yml

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-title: Recent search history for issues
-merge_request:
-author:

changelogs/unreleased/27271-missing-time-spent-in-issue-webhook.yml

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-title: Include time tracking attributes in webhooks payload
-merge_request: 9942
-author:

changelogs/unreleased/27293-remove-repeated-labels.yml

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-title: Remove duplicated tokens in issuable search bar
-merge_request:
-author:

changelogs/unreleased/27503-feature-status-aria-labels.yml

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-title: Add `aria-label` for feature status accessibility
-merge_request: 9830
-author:

changelogs/unreleased/27574-pipelines-empty-state.yml

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-title: Adds empty and error state to pipelines
-merge_request:
-author: