update happy and alex in Appveyor

[peterbecich]

#7033

I don't know what the old versions were.

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[jneira]

What is the motivation of the change (the linked issue has none)? did it fix something? In theory without the version, the "right" happy and alex versions for the ghc being used should be installed (Cabal should work for older ghc versions with different happy and alex versions and the ghc version used in appveyor could be different)

[Mikolaj]

We've just gutted out AppVeyor (#7604, too many false positives), which makes this PR spurious unless/until github actions croak (or get expensive) and we get back to AppVeyor.

@peterbecich: thank you for your work anyway!