ci: uprev kernel to 6.6.21
What does this MR do and why?
- Previous kernel used: 6.6.16 (except a618,a660 with 6.4 series)
- stable (upstream) changelog: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/log/?h=v6.6.21
- gfx-ci (downstream) changelog: https://gitlab.freedesktop.org/gfx-ci/linux/-/commits/v6.6-for-mesa-ci/?ref_type=HEADS
ci: uprev kernel to 6.6.21
Signed-off-by: David Heidelberg <david.heidelberg@collabora.com>/cc @robclark # for freedreno 6.4 -> 6.6.21