[Chore] Use LTS-21

[gromakovsky]

Description

This PR updated the Stackage resolver to the latest one and does some related cleaning.

Related issue(s)

Resolves #285

:white_check_mark: Checklist for your Pull Request

Ideally a PR has all of the checkmarks set.

If something in this list is irrelevant to your PR, you should still set this checkmark indicating that you are sure it is dealt with (be that by irrelevance).

Related changes (conditional)

• Tests

  • [x] If I added new functionality, I added tests covering it.
  • [x] If I fixed a bug, I added a regression test to prevent the bug from silently reappearing again.

• Documentation

  • [x] I checked whether I should update the docs and did so if necessary:
    • README
    • Haddock

• Public contracts

  • [x] Any modifications of public contracts comply with the Evolution of Public Contracts policy.
  • [x] I added an entry to the changelog if my changes are visible to the users and
  • [x] provided a migration guide for breaking changes if possible

Stylistic guide (mandatory)

• [x] My commits comply with the policy used in Serokell.
• [x] My code complies with the style guide.

✓ Release Checklist

• [ ] I updated the version number in package.yaml.
• [ ] I updated the changelog and moved everything under the "Unreleased" section to a new section for this release version.
• [ ] (After merging) I edited the auto-release.
  • Change the tag and title using the format vX.Y.Z.
  • Write a summary of all user-facing changes.
  • Deselect the "This is a pre-release" checkbox at the bottom.
• [ ] (After merging) I updated xrefcheck-action.
• [ ] (After merging) I uploaded the package to hackage.

[serokell-bot]

	3 Warnings
:warning:	In d012849441590c9c5890ec45e272f02f97076062 message lacks issue id: Automatically update flake.lock to the latest version. See also [Note].
:warning:	Description of d012849441590c9c5890ec45e272f02f97076062 does not follow the template. Try Problem:/Solution: structure. If you really have to, you can add And yes, I don't care about templates to the commit message body. See also [Note].
:warning:	Bad issue ID in branch name. Valid format for issue IDs: #123 or chore.

[Note]: Skip this check by adding wip, tmp or [temporary] to the commit subject. Fixup commits (marked with fixup! or squash!) are also exempt from this check.

Generated by :no_entry_sign: Danger

[Martoon-00]

CI is failing due to warnings / compilation errors also related to LTS-21. Other than that, LGTM.

[gromakovsky]

@Martoon-00 if you volunteer to fix them by any chance, feel free to go ahead :wink:

[Martoon-00]

Okay, I fixed the errors that I saw (note there are fixup commits, you may want to squash them after reviewing them).

However now some really scary errors bumped up, and I have to switch to other activities, so giving up on this right now.

[gromakovsky]

note there are fixup commits, you may want to squash them after reviewing them

Reviewed, squashed.

Also updated the resolver to a bit newer one, so that a newer minor GHC version is used. Let's see if it helps with any of scary errors. UPD: there are still some weird errors, not sure if it helped with any of them, but definitely not with all of them.

[serokell-bot]

	1 Error
:no_entry_sign:	Some fixup commits are still there.

Generated by :no_entry_sign: Danger

[int-index]

Superseded by #304