Build GHC in GHC2021

[Tom Ellis]

On Thu, Dec 07, 2023 at 12:53:02PM +0000, Richard Eisenberg wrote:
> I think this is an excellent idea! So excellent, that we've already done it. :)
> 
> When I try to compile with GHC 9.6.2 (what I have lying around), GHC2021 is in effect.
> 
> Is there something different you were thinking of?

I think Arnaud meant that compilations of GHC's codebase itself should
use the GHC2021 setting.