victoriametrics: retain for 4 years

Sounds overkill, but seeing my grafana for a year's metrics was quite
revealing. I'd like to keep that possibility

modules/monitoring/monitoring.nix

@@ -20,6 +20,7 @@ in
   services = {
     victoriametrics = {
       enable = true;
+      retentionPeriod = 48; # months. Should be enough, right?
     };
 
     prometheus = {