Update CHANGELOG.md for 9.4.0

CHANGELOG.md

...eleased/12151-add-since-and-until-params-to-issuables.yml

changelogs/unreleased/12200-add-french-translation.yml

changelogs/unreleased/13336-multiple-broadcast-messages.yml

changelogs/unreleased/18000-remember-me-for-oauth-login.yml

changelogs/unreleased/20628-add-oauth-implicit-grant.yml

changelogs/unreleased/23036-replace-dashboard-mr-spinach.yml

...nreleased/23036-replace-dashboard-new-project-spinach.yml

...elogs/unreleased/23036-replace-dashboard-todo-spinach.yml

changelogs/unreleased/23036-replace-snippets-spinach.yml

...llow-creation-of-files-and-dirs-with-spaces-in-web-ui.yml

changelogs/unreleased/23998-blame-age-map.yml

changelogs/unreleased/2501-ce-port-update-welcome-page.yml

changelogs/unreleased/25102-files-view-button.yml

...d/25103-mobile-members-page-user-avatar-is-misaligned.yml

...gelogs/unreleased/25164-disable-fork-on-project-limit.yml

changelogs/unreleased/26125-match-username-on-search.yml

...gelogs/unreleased/26212-upload-user-avatar-trough-api.yml

...released/27070-rename-slash-commands-to-quick-actions.yml

changelogs/unreleased/27586-center-dropdown.yml