Update dart/looker_sdk/lib/src/rtl/constants.dart

Co-authored-by: John Kaster <[email protected]>

Author: bryans99

dart/looker_sdk/lib/src/rtl/constants.dart

@@ -1,5 +1,5 @@
 const agentPrefix = 'DART-SDK';
-const defaultApiVersion = '3.1';
+const defaultApiVersion = '4.0';
 const lookerVersion = '21.21';
 const defaultTimeout = 120;
 final apiVersion = '4.0';