buffer: use Utf8LengthV2() instead of Utf8Length()

`Utf8Length()` is deprecated and was replaced during the V8 update, but
a subsequent commit introduced a new occurrence.

Refs: #58070
Refs: #58048
PR-URL: #58156
Reviewed-By: Gerhard Stöbich <deb2001-github@yahoo.de>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Yagiz Nizipli <yagiz@nizipli.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

Author: tniessen

src/node_buffer.cc

@@ -742,7 +742,7 @@ uint32_t FastByteLengthUtf8(
   Local<String> sourceStr = sourceValue.As<String>();
 
   if (!sourceStr->IsExternalOneByte()) {
-    return sourceStr->Utf8Length(isolate);
+    return sourceStr->Utf8LengthV2(isolate);
   }
   auto source = sourceStr->GetExternalOneByteStringResource();
   // For short inputs, the function call overhead to simdutf is maybe