Update CHANGELOG.md for 8.15.0

CHANGELOG.md

...gelogs/unreleased/15081-wrong-login-tab-ldap-frontend.yml

...elogs/unreleased/18435-autocomplete-is-not-performant.yml

changelogs/unreleased/18546-update-wiki-page-design.yml

...ogs/unreleased/19550-fix-contributer-graph-duplicates.yml

changelogs/unreleased/19620-auto-scroll-log.yml

changelogs/unreleased/19703-direct-link-pipelines.yml

changelogs/unreleased/20052-actions-table-vscroll.yml

changelogs/unreleased/20492-access-token-scopes.yml

changelogs/unreleased/22348-gitea-importer.yml

...ased/22373-reduce-queries-in-api-helpers-find_project.yml

changelogs/unreleased/22604-manual-actions.yml

...rse-install-rake-task-similar-to-gitlab-shell-install.yml

...gelogs/unreleased/22742-filter-protocol-relative-urls.yml

changelogs/unreleased/22781-user-generated-permalinks.yml

changelogs/unreleased/22849-ci-build-ref-slug.yml

changelogs/unreleased/22864-add-environment-slug.yml

changelogs/unreleased/22864-kubernetes-service.yml

changelogs/unreleased/23500-enable-colorvariable.yml

...e-common-helper-for-describe-pagination-params-in-api.yml