keyring: filter by region before checking version

In #14821 we fixed a panic that can happen if a leadership election happens in
the middle of an upgrade. That fix checks that all servers are at the minimum
version before initializing the keyring (which blocks evaluation processing
during trhe upgrade). But the check we implemented is over the serf membership,
which includes servers in any federated regions, which don't necessarily have
the same upgrade cycle.

Filter the version check by the leader's region.

.changelog/14901.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+keyring: Fixed a bug where keyring initialization is blocked by un-upgraded federated regions
+```

nomad/leader.go

@@ -1990,7 +1990,15 @@ func (s *Server) initializeKeyring(stopCh <-chan struct{}) {
 			return
 		default:
 		}
-		if ServersMeetMinimumVersion(s.serf.Members(), minVersionKeyring, true) {
+
+		members := s.serf.Members()
+		regionMembers := []serf.Member{}
+		for _, member := range members {
+			if member.Tags["region"] == s.Region() {
+				regionMembers = append(regionMembers, member)
+			}
+		}
+		if ServersMeetMinimumVersion(regionMembers, minVersionKeyring, true) {
 			break
 		}
 	}