Establish a plan for removing v1- commands

[mpickering]

It would be good to establish a concrete plan and timeline for removing v1- commands from cabal.

This has come up two years ago in #9109 and a year ago in #10262 but it's a very hard task to progress on without coming up with a proposal and overall plan.

If we start from the premise that we want to remove the v1-commands eventually, how do we get there?

I have made this ticket as I need something to track the task with but I'm planning to write down something more concrete and detailed about a possible way forward.

[ulysses4ever]

There's now a Cabal proposal related to this: haskell/cabal-proposals#4 /cc @andreasabel