Revert "disable EnablePublicChannelsMaterialization by default (#9418)" (#9427)

This reverts commit 5786b0d6d5, now that
the feature is safe to enable by default.

config/default.json

@@ -131,7 +131,7 @@
         "Trace": false,
         "AtRestEncryptKey": "",
         "QueryTimeout": 30,
-        "EnablePublicChannelsMaterialization": false
+        "EnablePublicChannelsMaterialization": true
     },
     "LogSettings": {
         "EnableConsole": true,