graphics/mesa: Update to 21.1.8

Update to the latest release of the 21.1 branch.

Release Notes: https://lists.freedesktop.org/archives/mesa-dev/2021-July/225455.html
Release Notes: https://lists.freedesktop.org/archives/mesa-dev/2021-August/225461.html
Release Notes: https://lists.freedesktop.org/archives/mesa-dev/2021-September/225465.html

Differential Revision: https://reviews.freebsd.org/D32351
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