Define GHC2024, but keep GHC2021 the default for now

Adam Gundryrequested to merge
wip/amg/ghc2024 into master

This is based on !11882 (closed), amended to keep GHC2021 as the default, and with a bit more overhaul of the documentation on language editions. Implements #24320 (closed) and fixes #24343 (closed).

Edited by Adam Gundry

Merge request reports