Update CHANGELOG.md for 9.1.0

[ci skip]

changelogs/unreleased/user-callout-showing-on-all-profiles.yml

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-title: User callout only shows on current users profile
-merge_request:
-author:

changelogs/unreleased/user-profile-join-date.yml

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-title: Removed the hours & minutes from the users start date on their profile
-merge_request:
-author:

changelogs/unreleased/zj-api-fix-build-events.yml

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-title: "Bugfix: POST /projects/:id/hooks and PUT /projects/:id/hook/:hook_id no longer ignore the the job_events param in the V4 API"
-merge_request: 10586
-author:

changelogs/unreleased/zj-chat-notification-default-branch.yml

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-title: Only send chat notifications for the default branch
-merge_request:
-author:

changelogs/unreleased/zj-fk-ci-triggers.yml

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-title: Add foreign key for ci_trigger_requests on ci_triggers
-merge_request: 10537
-author:

changelogs/unreleased/zj-kube-service-auto-fill.yml

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-title: Don't fill in the default kubernetes namespace
-merge_request:
-author: