graphics/mesa: Update to 21.3.6

Release notes for 21.3.5: https://lists.freedesktop.org/archives/mesa-dev/2022-January/225661.html
Release notes for 21.3.6: https://lists.freedesktop.org/archives/mesa-dev/2022-February/225673.html

MFH: 2022Q1
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D34239