Revert "[chore] Release 4.19.0 (#742)"

This reverts commit 3a86709.

Author: lahirumaramba

firebase.go

@@ -40,7 +40,7 @@ import (
 var defaultAuthOverrides = make(map[string]interface{})
 
 // Version of the Firebase Go Admin SDK.
-const Version = "4.19.0"
+const Version = "4.18.0"
 
 // firebaseEnvName is the name of the environment variable with the Config.
 const firebaseEnvName = "FIREBASE_CONFIG"