loki: Shorten retention period to 2 months to reduce space usage

We are running short on storage and don't really need 3 months of log,
so reduce it to 2 months instead of expanding the disk.

roles/loki/files/loki.yaml

@@ -38,5 +38,5 @@ compactor:
 
 limits_config:
   max_query_series: 10000
-  retention_period: 91d
-  max_query_lookback: 91d
+  retention_period: 61d
+  max_query_lookback: 61d