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/reflection/enclosing/lambdaInClassObject.kt

@@ -1,5 +1,5 @@
 class O {
-    class object {
+    default object {
         // Currently we consider <clinit> in class O as the enclosing method of this lambda,
         // so we write outer class = O and enclosing method = null
         val f = {}

compiler/testData/codegen/boxWithStdlib/reflection/genericSignature/kt5112.kt

@@ -5,13 +5,13 @@ class G<T>(val s: T) {
 }
 
 public trait ErrorsJvmTrait {
-    class object {
+    default object {
         public val param : G<String> = G("STRING")
     }
 }
 
 public class ErrorsJvmClass {
-    class object {
+    default object {
         public val param : G<String> = G("STRING")
     }
 }

compiler/testData/codegen/boxWithStdlib/reflection/genericSignature/kt6106.kt

@@ -6,7 +6,7 @@ open class B
 
 class A {
 
-    class object {
+    default object {
         [platformStatic]
         fun <T: B> a(s: T) : T {
             return s