[bug#54066] [PATCH]: Update mesa to 21.3.6 (fixup release)

[John Kehayias]

Thanks for the patch! I had just done a similar one locally and confirm that 
everything built fine on x86-64.

The last mesa update was part of core-updates(-frozen), but I heard in the past 
it went into staging. These days, at least, mesa moves quickly, updating every 
few weeks (the last few all 2 weeks or so apart).

So, I'd like to propose we set up a branch on the CI to build this mesa update 
to then push to master once we confirm no big breakage in building. Then we 
would have substitutes right away, which I think would be one of the big 
concerns with updating something like mesa (again, since this is a set of minor 
bugfix releases).

We could batch a few other changes to deploy at the same time, maybe the recent 
security grafts?

It would be great to leverage that CI to move quickly on updates like mesa 
which won't (we'll check!) be introducing breaking changes, merely lots of 
rebuilds. Likewise with grafts->updates, though I'm less familiar with what to 
look out for there. I suppose this is sort of the staging branch idea, but 
maybe a smaller scale and quicker. Mesa, for instance, would likely have a new 
version(s) if this is stretched over a month or two.

WDYT?