Added `kleidukos/get-tested action` to generate CI matrix

[DavidMazarro]

Thanks to the get-tested action, we can automatically generate the matrix of GHC versions to run the CI on. This keeps the tested-with versions we set in Cabal files accurate to what is actually being tested in the CI.

Since atomic-write was missing the tested-with stanza in its Cabal file, I also added that.