package/{mesa3d, mesa3d-headers}: bump version to 21.3.6

Release notes:
https://lists.freedesktop.org/archives/mesa-announce/2022-February/000663.html



Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>