remove cabal > 2.0 restriction on files in the 00-index file?

[gbaz]

This is a pretty ancient hack and imho there's no need to support cabal 1.x any more:

9a6be09

[andreasabel]

no need to support cabal 1.x any more

Ubuntu LTS 18.04 ships with cabal-install 1.24: https://packages.ubuntu.com/bionic/cabal-install

Will the 00-index be removed eventually altogether? Maybe that will be the right time to also dispose of the hack...

[gbaz]

LTS 18.04 is eol on april 30 2023, i think we can go ahead now.

I tend to think we need 00-index around for the "not setting up hackage security" option as there's no way to do an 01-index without jumping thru those hoops.