Update CHANGELOG.md for 9.1.0

[ci skip]

changelogs/unreleased/27878-new-service-for-creating-user.yml

----
-title: Implement user create service
-merge_request: 9220
-author: George Andrinopoulos

changelogs/unreleased/27910-admin-can-create-project-in-all-groups.yml

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-title: Allow admin to view all namespaces
-merge_request:
-author: George Andrinopoulos

changelogs/unreleased/27988-fix-transient-failure-in-commits-api.yml

----
-title: 'Add `requirements: { id: /.+/ }` for all projects and groups namespaced API
-  routes'
-merge_request: 9944
-author:

changelogs/unreleased/28030-infinite-offset.yml

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-title: allow offset query parameter for infinite list pages
-merge_request:
-author:

changelogs/unreleased/28187-project-name-cut-off-with-nested-groups.yml

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-title: Use toggle button to expand / collapse mulit-nested groups
-merge_request: 9501
-author:

changelogs/unreleased/28402-fix-starred-projects-filter-wrong-message-on-no-results.yml

----
-title: Fix wrong message on starred projects filtering
-merge_request:
-author: George Andrinopoulos

changelogs/unreleased/28424-labels-support-color-names-in-backend.yml

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-title: Labels support color names in backend
-merge_request: 9725
-author: Dongqing Hu

changelogs/unreleased/28494-mini-pipeline-graph-commit-view.yml

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-title: Adds pipeline mini-graph to system information box in Commit View
-merge_request:
-author:

changelogs/unreleased/28574-jira-trigers.yml

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-title: Remove confusing placeholder for JIRA transition_id
-merge_request:
-author:

changelogs/unreleased/28614-harmonious-color-palette.yml

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-title: Update color palette to a more harmonious and consistent one.
-merge_request: 10154
-author:

changelogs/unreleased/28634-todos-margin.yml

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-title: Remove extra margin at bottom of todos page
-merge_request:
-author:

changelogs/unreleased/28660-fix-dismissable-error-close-not-visible-enough.yml

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-title: Fixes dismissable error close is not visible enough
-merge_request: 9516
-author:

changelogs/unreleased/28695-move-all-associated-records-to-ghost-user.yml

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-title: Deleting a user should not delete associated records
-merge_request: 10467
-author:

changelogs/unreleased/28713-fe-style-guide.yml

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-title: Adds Frontend Styleguide to documentation
-merge_request: 9961
-author:

changelogs/unreleased/28732-expandable-folders.yml

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-title: Add back expandable folder behavior
-merge_request:
-author:

changelogs/unreleased/28799-todo-creation.yml

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-title: Create todos only for new mentions
-merge_request:
-author:

changelogs/unreleased/28810-projectfinder-should-handle-more-options.yml

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-title: ProjectsFinder should handle more options
-merge_request: 9682
-author: Jacopo Beschi @jacopo-beschi

changelogs/unreleased/28874-fix-milestone-issues-position-order-in-api.yml

----
-title: Order milestone issues by position ascending in api
-merge_request: 9635
-author: George Andrinopoulos

changelogs/unreleased/28899-linking-to-edit-file.yml

----
-title: Linking to blob edit page handles anonymous users and users without enough permissions
-  to edit directly
-merge_request:
-author:

changelogs/unreleased/28991-viewing-old-wiki-page-version-edit-button-exists.yml

----
-title: When viewing old wiki page version, edit button should be disabled
-merge_request: 9966 
-author: TM Lee