Update CHANGELOG.md for 12.8.0

changelogs/unreleased/sh-update-acme-client.yml

changelogs/unreleased/sh-use-cte-for-projects-finder.yml

...unreleased/show-kubernetes-namespace-on-job-show-page.yml

changelogs/unreleased/show_selected_template_type.yml

...gelogs/unreleased/ss-fix-text-truncated-on-board-list.yml

changelogs/unreleased/sy-sync-issues-with-sentry.yml

changelogs/unreleased/sy-sync-plugin-sentry-issues.yml

changelogs/unreleased/tc-create-project-settings.yml

changelogs/unreleased/tc-fix-38088.yml

changelogs/unreleased/tidy-cluster-advanced-settings.yml

...logs/unreleased/toggle-analytics-navbar-feature-flags.yml

changelogs/unreleased/tr-fix-frontend-polling.yml

changelogs/unreleased/turn-on-merged-branch-cache-flag.yml

changelogs/unreleased/unauthaccess-to-public-keys.yml

changelogs/unreleased/update-geo-node-service.yml

.../unreleased/update-gitlab-runner-helm-chart-to-0-13-0.yml

.../unreleased/update-gitlab-runner-helm-chart-to-0-13-1.yml

changelogs/unreleased/update-graphicsmagick-to-1-3-34.yml

changelogs/unreleased/update-handlebarsjs.yml

changelogs/unreleased/update_cluster-applications_v0-7-0.yml