Migrate CI to use stack 2.1

[alanz]

At some point we need to migrate CI to use stack 2.1

But I think we should keep at least one build using 1.9.3, to make sure we don't break backward compatibility.

[jneira]

it seems azure and circleci (with the stack upgrade in the docker file? https://hub.docker.com/r/alanz/haskell-hie-ci/dockerfile) and travis (cause it uses homebrew) uses stack 2.1.
@alanz maybe we could close this one

[nponeccop]

we should keep at least one build using 1.9.3

We should at least document which builds use stack 1.9.x

[fendor]

I think, currently all CI's are using stack 2.1.3.1 or something like that?

[fendor]

Closed by accident

[jneira]

We are validating in the install script that stack version is greater than 2.1.1 and it is listed in README as one of the requirements to build so in fact we've deprecated stack-1.9.3 to build hie