Update CHANGELOG.md for 12.8.0

changelogs/unreleased/refactoring-entities-file-8.yml

changelogs/unreleased/refactoring-entities-file-9.yml

changelogs/unreleased/refactoring-entities-file-final.yml

changelogs/unreleased/refactoring-entities-file.yml

changelogs/unreleased/remove-issue_link_types.yml

changelogs/unreleased/remove-multiple-milestone-mess.yml

...gelogs/unreleased/remove-self-monitoring-feature-flag.yml

changelogs/unreleased/remove_milestone_id_from_epics3.yml

changelogs/unreleased/rename-self-monitoring-project.yml

changelogs/unreleased/reorder-signup-omniauth-options.yml

changelogs/unreleased/reschedule-issue-tracker-migration.yml

changelogs/unreleased/resolve_gitlab_issue_196651.yml

changelogs/unreleased/resolve_gitlab_issue_196658.yml

changelogs/unreleased/rest_api_log_last_activity_on.yml

...revert_rename_services_template_to_instance_migration.yml

changelogs/unreleased/rk4bir-master-patch-88996.yml

changelogs/unreleased/rs-scheduled-pipeline-play-api.yml

changelogs/unreleased/rubyzip-qa-update.yml

...gelogs/unreleased/sarnold-197129-graphql-feature-flag.yml

changelogs/unreleased/sdesk-name-handler.yml