GHC 9.0 CI jobs

[fgaz]

I'm a bit confused about how this is done. I guess by modifying the files in templates cabal-dev-scripts/src/GenValidate.hs and running make github-actions... but those files say that they are generated too. Maybe it's like in cabal-install.cabal.zinza: the warning is there, but only because it has to go in the generated files somehow.

We probably want to document all this

/cc @emilypi

[Mikolaj]

While we are at it, we can also remove the hack at

cabal/cabal.project

Lines 23 to 24 in 3c4023f

	allow-newer:
	hackage-security:Cabal

to avoid publishing something depending on allow-newer.

l will bump base bounds of hackage-security as needed.

[andreabedini]

@fgaz @Mikolaj is this still relevant?

[Mikolaj]

Good find. Not having any allow-newer in CI is relevant, but that's it. Closing.