Draft: [#919] Update stack resolver to lts-20 (wip)

Description

This MR updates stack resolver to lts-20, and consequently GHC to 9.2.7.

There are a few unfortunate regressions in GHC 9.2 however, so it's up in the air whether we want to make the switch now or wait for a bit.

UPD: The most annoying issues were fixed, some minor quirks remain. However, now haskell.nix is apparently borked

UPD2: GHC 9.2.7 segfaults in some cases. I did manage to work around that (yay), but that's likely a no-go for us anyway.

Related issue(s)

Resolves #919 (closed)

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