Bump Cabal versions from 2.5 -> 3.0

[hvr]

NOTE: This will most certainly fail at CI because of the cabal-testsuite.cabal requiring a bleeding edge lib:Cabal for its Setup.hs -- I can't find the ticket, but there's a a plan to decouple cabal-testsuite from requiring a bleeding edge lib:Cabal (UPDATE: see #5916)

Another (less ideal) workaround would be to use a cabal 3.0.* executable for building cabal-testsuite

Addressing #5819

[phadej]

cabal-testsuite indeed failed on appveyor

[bgamari]

On what sort of timeframe can we expect this to be merged? Perhaps it would make sense for GHC to just start tracking this branch?

[hvr]

@bgamari we could push the commit from this PR together with a instant-revert-followup-commit to master, and then you could refer to that; using this branch would be dangerous as there's no guarantee the commit 971f083 will remain reachable and then you'd end up with a broken gitlink in ghc.git

[bgamari]

@hvr, we could do that but we'll need to sort out #16378 before we can do so.

[hvr]

@bgamari can that very backpack failure be reproduced with GHC 8.6? or is it really a function of the GHC version and specific to GHC 8.8?

[23Skidoo]

This is green, time to merge?