Use alex-3.2.7.3
See ghc!11274 (comment 526630) and ghc#22356 (closed). To generate Cabal lexer working under JS backend, we should use alex-3.2.7.3
or newer (see https://hackage.haskell.org/package/alex-3.4.0.0/changelog).
I'm not sure if this PR is the right way to enforce it, I'm just cargo culting a9c0f5ef.
CC @mpickering