* ReleaseNotes: Updated changelog and release notes for 7.4.0-beta1

* Apply suggestions from code review

Co-authored-by: Ursula Kallio <73951760+osg-grafana@users.noreply.github.com>

Co-authored-by: Giordano Ricci <gio.ricci@grafana.com>
Co-authored-by: Ursula Kallio <73951760+osg-grafana@users.noreply.github.com>