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()<!> {}
     }
 }

compiler/testData/diagnostics/testsWithStdLib/duplicateJvmSignature/platformStaticInClassObject.txt

@@ -15,7 +15,7 @@ internal final class Derived : Base {
     public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
     public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
 
-    internal class object Default {
+    internal default object Default {
         private constructor Default()
         public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
         kotlin.platform.platformStatic() internal final fun foo(): kotlin.Unit