Update CHANGELOG.md for 8.15.0

changelogs/unreleased/abuse_report-fixture.yml

changelogs/unreleased/adam-auto-deploy.yml

changelogs/unreleased/add_info_to_qr.yml

changelogs/unreleased/allow-more-filenames.yml

changelogs/unreleased/api-branch-status.yml

changelogs/unreleased/api-cherry-pick.yml

changelogs/unreleased/api-delete-group-share.yml

changelogs/unreleased/api-expose-commiter-details.yml

changelogs/unreleased/api-remove-source-branch.yml

changelogs/unreleased/api-simple-group-project.yml

changelogs/unreleased/awards_handler.yml

changelogs/unreleased/bitbucket-oauth2.yml

changelogs/unreleased/cache-last-commit-sha-for-path.yml

changelogs/unreleased/change_development_build_fixtures.yml

changelogs/unreleased/chomp-git-status-message.yml

changelogs/unreleased/cleanup-common_utils-js.yml

changelogs/unreleased/comments-fixture.yml

...logs/unreleased/create-dynamic-fixture-for-build_spec.yml

changelogs/unreleased/destroy-session.yml

changelogs/unreleased/dev-issue-24554.yml