cryptol-remote-api: Allow building with aeson-2.0.*

[RyanGlScott]

This introduces some CPP, unfortunately, but this probably necessary to
continue allowing cryptol-remote-api to be buildable with old versions of
aeson until the ecosystem catches up with aeson-2.0.*.

[pnwamk]

LGTM.

Out of curiosity, why is this an improvement over leaving the upper bound at 2.0 until the ecosystem widely uses @aeson-2.0.0.0@ or later?

[RyanGlScott]

Out of curiosity, why is this an improvement over leaving the upper bound at 2.0 until the ecosystem widely uses @aeson-2.0.0.0@ or later?

We could do that, I suppose. Supporting aeson-2.0.* is a prerequisite to being able to build things with GHC 9.2, however, and I'm not sure whether we'll support 9.2 before or after the ecosystem catches up to aeson-2.0.*. If I had to place a bet, I'd bet "before".