Update CHANGELOG.md for 8.16.0

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

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

changelogs/unreleased/fix-import-users.yml

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

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

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

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

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

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

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

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

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

changelogs/unreleased/fix_auto_merging.yml

changelogs/unreleased/get_last_used_date_of_ssh_key.yml

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

changelogs/unreleased/input-button-hover.yml

changelogs/unreleased/issue-boards-animate.yml

changelogs/unreleased/issue_22664.yml

changelogs/unreleased/issue_25017.yml

changelogs/unreleased/issue_25578.yml