Update CHANGELOG.md for 8.16.0

[ci skip]

changelogs/unreleased/single-edit-comment-widget-2.yml

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-title: Refactored note edit form to improve frontend performance on MR and Issues
-  pages, especially pages with has a lot of discussions in it
-merge_request: 8356
-author:

changelogs/unreleased/speed-up-dashboard-milestone-index.yml

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-title: Speed up dashboard milestone index by scoping IssuesFinder to user authorized
-  projects
-merge_request: 8524
-author:

changelogs/unreleased/support-google-cloud-storage-backups.yml

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-title: Re-add Google Cloud Storage as a backup strategy
-merge_request:
-author:

changelogs/unreleased/time-tracking-api.yml

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-title: Add new endpoints for Time Tracking.
-merge_request: 8483
-author:

changelogs/unreleased/update-gitlab-markup-gem.yml

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-title: Update the gitlab-markup gem to the version 1.5.1
-merge_request: 8509
-author:

changelogs/unreleased/validate-title-length.yml

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-title: "Validate label's title length"
-merge_request: 5767
-author: Tomáš Kukrál

changelogs/unreleased/view-ce-vs-ee.yml

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-title: About GitLab link in sidebar that links to help page
-merge_request: 8316
-author:

changelogs/unreleased/zj-unadressable-url-variables.yml

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-title: Don't validate environment urls on .gitlab-ci.yml
-merge_request:
-author: