Update CHANGELOG.md for 9.1.0

[ci skip]

changelogs/unreleased/tc-fix-pipeline-recipient.yml

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-title: Clearly show who triggered the pipeline in email
-merge_request: 10283
-author:

changelogs/unreleased/tc-fix-unplayable-build-action-404.yml

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-title: Disable pipeline and environment actions that are not playable
-merge_request: 10052
-author:

changelogs/unreleased/tc-pipeline-show-trigger-date.yml

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-title: Show correct user & creation time in heading of the pipeline page
-merge_request: 9936
-author:

changelogs/unreleased/tc-show-pipeline-coverage-if-avail.yml

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-title: Show the build/pipeline coverage if it is available
-merge_request:
-author:

changelogs/unreleased/time-tracking-color-not-consistent.yml

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-title: Corrected time tracking icon color in the issuable side bar
-merge_request:
-author:

changelogs/unreleased/update-issue-board-cards-design.yml

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-title: Update issue board cards design
-merge_request: 10353
-author:

changelogs/unreleased/update-test-bundle-ignored-files.yml

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-title: update test_bundle.js ignored files
-merge_request:
-author:

changelogs/unreleased/usage-ping-port.yml

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-title: Add usage ping to CE
-merge_request:
-author:

changelogs/unreleased/use-corejs-polyfills.yml

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-title: Standardize on core-js for es2015 polyfills
-merge_request: 9749
-author:

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-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: