[#919] LTS-21

Description

Update to LTS-21.

At present, this uses allow-newer: true. We need to update quite a few upstream packages (many our own).

Good news, this seems to work okay. Had to get very creative with custom type errors for scopes, but the solution I ultimately came up with seems to work pretty well, at least here.

Sorry everything's in one giant commit, since this is based off prior work wrt LTS-20 and GHC-9.6, separating all this logically wasn't really feasible.

Related issue(s)

Resolves #919 (closed)

✅ Checklist for your Merge Request

Related changes (conditional)

• Tests (see short guidelines)

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

• Documentation

  • I checked whether I should update the docs and did so if necessary:
    • Root README and other README.md files
    • Haddock
    • docs/
  • I updated changelog files of all affected packages released to Hackage if my changes are externally visible.

Stylistic guide (mandatory)

• My commits comply with the following policy.
• My code complies with the style guide.