Update CHANGELOG.md for 8.15.0

changelogs/unreleased/24927-custom-event-polyfill-test.yml

...n-controls-for-merge-request-diff-containing-an-image.yml

...released/24982-ux-improvement-sign-in-success-message.yml

changelogs/unreleased/24999-fix-project-avatar-alignment.yml

changelogs/unreleased/25002-sentence-case-dashboard-tabs.yml

...hoverstates-for-collapsed-issue-merge-request-sidebar.yml

...gs/unreleased/25026-authenticate-user-for-new-snippet.yml

...s/unreleased/25031-do-not-raise-error-in-autocomplete.yml

...unreleased/25093-hide-new-issue-btn-non-loggedin-user.yml

.../unreleased/25098-header-margins-on-pipeline-settings.yml

...nreleased/25106-hide-issue-mr-button-for-not-loggedin.yml

changelogs/unreleased/25136-last-deployment-link.yml

...h-command-for-issue-create-needs-better-documentation.yml

...elogs/unreleased/25202-fix-mr-widget-content-wrapping.yml

changelogs/unreleased/25207-text-overflow-env-table.yml

...ogs/unreleased/25221-fix-build-status-overflow-mobile.yml

...template-error-undefined-method-text-for-nil-nilclass.yml

changelogs/unreleased/25264-ref-commit.yml

...gelogs/unreleased/25272_fix_comments_tab_disappearing.yml

changelogs/unreleased/25294-remove-signed-out-msg.yml