disable EnablePublicChannelsMaterialization by default (#9418)

* disable EnablePublicChannelsMaterialization by default

Creating triggers on Amazon RDS seems to reqiure extra privileges. More
investigation is required.

* update model/config.go too

model/config.go

@@ -687,7 +687,7 @@ func (s *SqlSettings) SetDefaults() {
 	}
 
 	if s.EnablePublicChannelsMaterialization == nil {
-		s.EnablePublicChannelsMaterialization = NewBool(true)
+		s.EnablePublicChannelsMaterialization = NewBool(false)
 	}
 }