Update RELEASE.md to reflect mailing list change (#5472)
Update RELEASE.md to reflect mailing list change (#5472)

We are not using `prometheus-users@googlegroups.com` for announcments
anymore.

Signed-off-by: Bjoern Rabenstein <a31805f6001ab3b535ea95133fbc4db46e97e8d1@rabenste.in>
alileza/prometheus,kangwoo/prometheus,Gouthamve/prometheus,keegancsmith/prometheus,GoogleCloudPlatform/prometheus,GoogleCloudPlatform/prometheus,alileza/prometheus,GoogleCloudPlatform/prometheus,Gouthamve/prometheus,prometheus/prometheus,prometheus/prometheus,alileza/prometheus,alileza/prometheus,keegancsmith/prometheus,kangwoo/prometheus,keegancsmith/prometheus,Gouthamve/prometheus,prometheus/prometheus,Gouthamve/prometheus,keegancsmith/prometheus,prometheus/prometheus,kangwoo/prometheus,GoogleCloudPlatform/prometheus,keegancsmith/prometheus,prometheus/prometheus,alileza/prometheus,kangwoo/prometheus,GoogleCloudPlatform/prometheus