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/diagnostics/testsWithStdLib/duplicateJvmSignature/platformStaticInClassObject.kt

@@ -5,7 +5,7 @@ open class Base {
 }
 
 class Derived : Base() {
-    class object {
+    default object {
         <!ACCIDENTAL_OVERRIDE!>platformStatic fun foo()<!> {}
     }
 }