src: remove usage of std::shared_ptr<T>::unique()

`std::shared_ptr<T>::unique()` has been removed in C++20, so this change
uses `std::shared_ptr<T>::use_count()` instead which is available in
C++20.

Fixes: https://github.com/nodejs/node/issues/47311
Signed-off-by: Darshan Sen <raisinten@gmail.com>
PR-URL: https://github.com/nodejs/node/pull/47315
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Jiawen Geng <technicalcute@gmail.com>
Reviewed-By: Yagiz Nizipli <yagiz@nizipli.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>

src/node_threadsafe_cow-inl.h

@@ -7,7 +7,7 @@ namespace node {
 
 template <typename T>
 T* CopyOnWrite<T>::write() {
-  if (!data_.unique()) {
+  if (data_.use_count() > 1l) {
     data_ = std::make_shared<T>(*data_);
   }
   return data_.get();