loki: Retention period should be a multiple of periodic table duration

Fixes: 49091634 ("loki: Bump the retention period to 3 months")

roles/loki/files/loki.yaml

@@ -56,8 +56,8 @@ limits_config:
   max_query_series: 10000
 
 chunk_store_config:
-  max_look_back_period: 2190h
+  max_look_back_period: 2184h
 
 table_manager:
   retention_deletes_enabled: true
-  retention_period: 2190h  # 91,25 days
+  retention_period: 2184h  # 91 days