Increase data retention to 5 years

Now that we have more space, we can extend the retention time to 5 years
to hold the monitoring metrics. For a year we have:

	# du -sh /var/lib/prometheus2
	13G     /var/lib/prometheus2

So we can expect it to increase to about 65 GiB. In the future we may
want to reduce some adquisition frequency.

Reviewed-by: Aleix Boné <abonerib@bsc.es>

m/hut/monitoring.nix

@@ -46,7 +46,7 @@
   services.prometheus = {
     enable = true;
     port = 9001;
-    retentionTime = "1y";
+    retentionTime = "5y";
     listenAddress = "127.0.0.1";
   };