mingw-w64: 5.0.4 -> 6.0.0

Version 6.0.0 brings better Win32 API coverage and bugfixes.
It's been used in various distros long enough to be considered stable.
Latest version 7.0.0 hasn't received extensive testing yet.

Announce mail:
https://sourceforge.net/p/mingw-w64/mailman/message/36416777/

pkgs/os-specific/windows/mingw-w64/headers.nix

@@ -4,8 +4,6 @@ stdenvNoCC.mkDerivation {
   name = "${mingw_w64.name}-headers";
   inherit (mingw_w64) src meta;
 
-  patches = [ ./osvi.patch ];
-
   preConfigure = ''
     cd mingw-w64-headers
   '';