Replace 'class object' with 'default object' in renderers and test data

Includes changes to decompiled text
Old syntax is used in builtins and project code for now

compiler/testData/codegen/boxWithStdlib/native/privateStatic.kt

@@ -2,7 +2,7 @@ import kotlin.jvm.*
 import kotlin.platform.*
 
 class C {
-    class object {
+    default object {
         private platformStatic native fun foo()
     }