Add Stack setup for GHC 9.2.8

[mmhat]

This PR adds a stack.yaml for LTS 20, i.e. GHC 9.2.8. This one is now used by default but we run an additional CI job with the LTS 18 setup to ensure that Dhall builds for both GHCs. Additional changes:

• Bumped GHC 8.10 stack setup to LTS 18.28.
• Require lsp>=1.5.0.0 in dhall-lsp-server. This made the setup for GHC 9.2.8 easier.
• Added kind signatures in Dhall.Syntax.Expr in order to fix doctests.
• Updated the use GitHub Actions. In particular, use the new Haskell action (since the old one is deprecated).

[mmhat]

Apparently Hydra does not like the new text-rope dependency: https://hydra.dhall-lang.org/eval/21496896#tabs-errors
I am not really familiar with the Nix setup (and I am also not a Nix user) so a little help here would be highly appreciated.

[mmhat]

Ok, I think the issue with Hydra is the last missing thing here - but as already mentioned I don't know how to fix that on my own. Pinging @Gabriella439 , since she probably knows the setup best.

As a side note, I think this work here supersedes the one in #2428 (which I discovered too late).

Also, if we merge this we should adjust the required status checks for the main branch to match the new names of the jobs.

[Gabriella439]

ah, whoops, it's because of this issue:

https://hydra.dhall-lang.org/jobset/dhall-haskell/2531#tabs-errors

error: anonymous function at …/default.nix:1:1 called without required argument 'text-rope'

If you do cabal2nix cabal://text-rope > ./nix/packages/text-reop.nix I think that should fix it or at least give you a new error. I'd do it myself but I don't have push rights to your branch (also, for future reference, you can push your branches to this repository if you want me to be able to push fixes to them for you).

[Vekhir]

@mmhat There was a possible solution suggested by @Gabriella439, did you investigate if it solves the issue, so this PR can be merged?

Also, this PR doesn't fully address the tests, where lsp-test < 0.15 is specified (0.16 is the newest version). Have you looked at that? Else I'd do that.

[mmhat]

@Vekhir Thanks for the ping! I think lsp-test < 0.15 is fine: All lsp-test version that are newer require lsp >= 2.0 which we exclude in our version constraints; IMHO we should relax the lsp-test upper bound once we moved to lsp >= 2.0.

@Gabriella439 Thanks a lot for the hint; That totally worked out! I took the liberty to merge the branch and adjust the required GitHub status checks accordingly. I also deleted the stale Travis CI webhook that didn't work anyway.