Rename Extension annotation to ExtensionFunctionType

The old one is kept temporarily and will be removed later

core/builtins/src/kotlin/Annotations.kt

@@ -69,6 +69,11 @@ public enum class DeprecationLevel {
  */
 @Target(TYPE)
 @MustBeDocumented
+public annotation class ExtensionFunctionType
+
+@Target(TYPE)
+@MustBeDocumented
+@Deprecated("Use @ExtensionFunctionType instead.", replaceWith = ReplaceWith("@ExtensionFunctionType"))
 public annotation class Extension
 
 /**