Migrate repository to haskell org?

Historically, GHCup has provided more stack binaries than upstream: https://downloads.haskell.org/~ghcup/unofficial-bindists/stack/2.11.1/

This mainly is about Darwin M1. I've stopped doing that, because it's too much work.

@angerman and me have set up private Github runners for Darwin M1 and linux aarch64. However, these can only be shared amongst the github haskell org.

If stack would migrate the main repo there, then we could share those runners. This would change nothing in terms of ownership.

Opinions @mpilgrem @snoyberg @cdornan @david-christiansen @chreekat