Upgrade LTS (upgrades GHC to 8.6.5)

[bbarker]

I tested this and spago could build and run. This helps keep it in line with the current version of GHC available in NixOS as well.

Description of the change

Upgrade Stackage LTS version and GHC version.

Checklist:

• Added the change to the "Unreleased" section of the changelog
• Added some example of the new feature to the README
• Added a test for the contribution (if applicable)

P.S.: the above checks are not compulsory to get a change merged, so you may skip them. However, taking care of them will result in less work for the maintainers and will be much appreciated 😊

[f-f]

@bbarker thanks for this! We already upgraded to GHC 8.6 once, but had to downgrade again in #479 for compatibility reasons.

So I'd like to keep Spago on 8.4 until that's in the range of the last three versions of the compiler with an LTS (which is usually the moment when packages start dropping bounds), which means that I'd like to upgrade once there is an LTS with GHC 8.10 out.

[arrowd]

I'm trying to build Spago with GHC 8.8.3. Judging from this thread, it is completely unsupported?

The error I got is

src/Spago/Dhall.hs:32:12: error:
    Not in scope: data constructor ‘Dhall.Format.Check’
    Perhaps you meant variable ‘Dhall.Format.input’ (imported from Dhall.Format)
    Module ‘Dhall.Format’ does not export ‘Check’.
   |
32 |   try (f $ Dhall.Format.Check path) >>= \case
   |            ^^^^^^^^^^^^^^^^^^

src/Spago/Dhall.hs:34:11: error:
    Not in scope: data constructor ‘Dhall.Format.Modify’
    Perhaps you meant one of these:
      ‘Dhall.Format.Format’ (imported from Dhall.Format),
      variable ‘Dhall.Format.format’ (imported from Dhall.Format)
    Module ‘Dhall.Format’ does not export ‘Modify’.
   |
34 |       f $ Dhall.Format.Modify path
   |           ^^^^^^^^^^^^^^^^^^^

[f-f]

@arrowd it sounds like you're trying to build with dhall-1.30? We're still at dhall-1.29, but I'd welcome a PR to upgrade to the latest. However, Dhall itself might not build with 8.8 yet, see dhall-lang/dhall-haskell#1249

[arrowd]

Yep, downgrading dhall to 1.29 allowed me to build spago successfully.

[f-f]

@bbarker since we're not looking to upgrade LTS any time soon, I'll just close this PR for now, to keep the backlog tidy