Update CHANGELOG.md for 8.16.0

[ci skip]

changelogs/unreleased/fix-copy-issues-empty-state.yml

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-title: Improve copy in Issue Tracker empty state
-merge_request: 8202
-author:

changelogs/unreleased/fix-external-status-badge-links.yml

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-title: Link external build badge to its target URL
-merge_request: 8611
-author:

changelogs/unreleased/fix-import-users.yml

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-title: Fix import/export wrong user mapping
-merge_request: 
-author: 

changelogs/unreleased/fix-keep-artifacts-button-visibility.yml

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-title: Hide build artifacts keep button if operation is not allowed
-merge_request: 8501
-author:

changelogs/unreleased/fix-light-hr-in-descriptions.yml

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-title: Darkened hr border color in descriptions because of update of bootstrap
-merge_request: 8333
-author:

changelogs/unreleased/fix-more-orphans-remove-undeleted-groups.yml

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-title: Remove extra orphaned rows when removing stray namespaces
-merge_request: 7841
-author: 

changelogs/unreleased/fix-no-milestone-option-for-projects-endpoint-23194.yml

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-title: 'API: fix query response for `/projects/:id/issues?milestone="No%20Milestone"`'
-merge_request: 8457
-author: Panagiotis Atmatzidis, David Eisner

changelogs/unreleased/fix-pipeline-ref-path-serialization.yml

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-title: Do not generate pipeline branch/tag path if not present
-merge_request: 8658
-author:

changelogs/unreleased/fix-project-delete-tooltip.yml

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-title: Fix project queued for deletion re-creation tooltip
-merge_request: 
-author: 

changelogs/unreleased/fix-serialized-commit-path.yml

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-title: Fix links to commits pages on pipelines list page
-merge_request: 8558
-author:

changelogs/unreleased/fix-timezone-due-date-picker.yml

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-title: Fix date inconsistency on due date picker
-merge_request: 7422
-author: Giuliano Varriale

changelogs/unreleased/fix-user-api-confirm-param.yml

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-title: Fix 500 error when POSTing to Users API with optional confirm param
-merge_request:
-author:

changelogs/unreleased/fix_auto_merging.yml

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-title: Fix Merge When Pipeline Succeeds immediate merge bug
-merge_request: 8685
-author:

changelogs/unreleased/get_last_used_date_of_ssh_key.yml

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-title: Record and show last used date of SSH Keys
-merge_request: 8113
-author: Vincent Wong

changelogs/unreleased/i--25814-500-error.yml

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-title: Fix Compare page throws 500 error when any branch/reference is not selected
-merge_request: 8492
-author: Martin Cabrera

changelogs/unreleased/input-button-hover.yml

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-title: Add hover state to MR comment reply button
-merge_request:
-author:

changelogs/unreleased/issue-boards-animate.yml

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-title: Added animations to issue boards interactions
-merge_request:
-author:

changelogs/unreleased/issue_22664.yml

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-title: Check if user can read project before being assigned to issue
-merge_request:
-author:

changelogs/unreleased/issue_25017.yml

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-title: Show 'too many changes' message for created merge requests when they are too large
-merge_request:
-author:

changelogs/unreleased/issue_25578.yml

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-title: Fix redirect after update file when user has forked project
-merge_request:
-author: