ObjC: Block Roots from being created.

It has never made sense to create instances (all methods are class methods), just overlooked catching it at compile time until now.

PiperOrigin-RevId: 891804535

Author: thomasvl

objectivec/GPBRootObject.h

@@ -18,6 +18,9 @@ NS_ASSUME_NONNULL_BEGIN
  **/
 @interface GPBRootObject : NSObject
 
+- (instancetype)init NS_UNAVAILABLE;
++ (instancetype)new NS_UNAVAILABLE;
+
 /**
  * @return An extension registry for the given file and all the files it depends
  * on.