Update CHANGELOG.md for 8.16.0

...oject-commits-commits-does-not-work-with-project-path.yml

changelogs/unreleased/26352-user-dropdown-settings.yml

...elogs/unreleased/26435-show-project-avatars-on-mobile.yml

...eased/26445-make-icon-buttons-accessible-via-keyboard.yml

...released/26446-access-download-artifacts-via-keyboard.yml

changelogs/unreleased/26472-math-margin.yml

changelogs/unreleased/26504-mr-discussion-btn.yml

...s/unreleased/26587-metrics-middleware-endpoint-is-nil.yml

changelogs/unreleased/26601-dropdown-makes-request-close.yml

changelogs/unreleased/26615-pipeline-status-cell.yml

...ogs/unreleased/26616-fix-search-group-project-filters.yml

...ogs/unreleased/26667-pipeline-width-for-huge-pipeline.yml

...released/26773-fix-project-statistics-repository-size.yml

...es-issue-boards-list-colored-top-border-visual-glitch.yml

changelogs/unreleased/8623-correct-robots-txt.yml

...s/unreleased/add-changelog-search-bar-first-iteration.yml

changelogs/unreleased/add_email_password_confirmation.yml

...unreleased/additional-award-emoji-repositioning-fixes.yml

changelogs/unreleased/allow_plus_sign_for_snippets.yml

changelogs/unreleased/asciidoctor-plantuml.yml