Add Gerrit v3.9.4 release notes

Also specify that v3.9.3 has been withdrawn and amend the link
to Change 411279, as it was labelled as issue number.

Update links to the latest Gerrit stable release, now
pointing to v3.9.4.

Change-Id: I056dcbf6620b76bca70d74e60b4c092d199ca6cc
5 files changed