make "public_portals: false" the default

Co-authored-by: Tulir Asokan <tulir@maunium.net>

mautrix_signal/example-config.yaml

@@ -136,7 +136,7 @@ bridge:
     autocreate_contact_portal: false
     # Whether or not to make portals of Signal groups in which joining via invite link does
     # not need to be approved by an administrator publicly joinable on Matrix.
-    public_portals: true
+    public_portals: false
     # Whether or not to use /sync to get read receipts and typing notifications
     # when double puppeting is enabled
     sync_with_custom_puppets: true