Update libpng 1.6.43 for Qt

We need to make sure all security critical 3rd party modules are the latest one before we can release. It seems that libpng needs an update; we are using 1.6.42 but 1.6.43 is already available:

http://www.libpng.org/pub/png/libpng.html

and libpng is marked as security critical one.

This is needed as soon as possible especially for Qt 6.2.12.