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/tests/classObjects/classObjectHeader.kt

@@ -4,6 +4,6 @@ open class ToResolve<SomeClass>(<!UNUSED_PARAMETER!>f<!> : (Int) -> Int)
 fun testFun(<!UNUSED_PARAMETER!>a<!> : Int) = 12
 
 class TestSome<P> {
-    class object : ToResolve<<!INACCESSIBLE_OUTER_CLASS_EXPRESSION!>P<!>>({testFun(it)}) {
+    default object : ToResolve<<!INACCESSIBLE_OUTER_CLASS_EXPRESSION!>P<!>>({testFun(it)}) {
     }
 }