Fix 'redaction_retention_period' sampel config to match guidelines

docs/sample_config.yaml

@@ -314,7 +314,7 @@ listeners:
 #
 # Defaults to `7d`. Set to `null` to disable.
 #
-redaction_retention_period: 7d
+#redaction_retention_period: 28d
 
 # How long to track users' last seen time and IPs in the database.
 #

synapse/config/server.py

@@ -742,7 +742,7 @@ class ServerConfig(Config):
         #
         # Defaults to `7d`. Set to `null` to disable.
         #
-        redaction_retention_period: 7d
+        #redaction_retention_period: 28d
 
         # How long to track users' last seen time and IPs in the database.
         #