burp: Store backups for 4 weeks + 3 month

Signed-off-by: Maximilian Wilhelm <max@sdn.clinic>

burp/server/burp-server.conf.tmpl

@@ -64,8 +64,8 @@ version_warn = 1
 # For example, if running backups daily, setting 7, 4, 6 will keep
 # 7 daily backups, 4 weekly, and 6 four-weekly backups.
 keep = 7
-# keep = 4
-# keep = 6
+keep = 4
+keep = 3
 
 # Run as different user/group.
 # user=graham