Allow transformers 0.6

[sjakobi]

https://hackage.haskell.org/package/transformers-0.6.0.0/changelog

---

Build command as of 2022-05-10:

cabal build --constraint 'transformers >= 0.6' -w ghc-8.10 --allow-newer=websockets-snap:mtl,io-streams:transformers,snap-server:mtl,io-streams-haproxy:transformers,snap-core:mtl,snap-core:transformers

Blocked on:

• [x] https://github.com/haskell/mtl/issues/88 (via websockets-snap)
• [ ] https://github.com/snapframework/snap-server/issues/144
• [x] https://github.com/ekmett/exceptions/issues/82
• [ ] https://github.com/snapframework/snap-core/issues/313

Bounds only (I guess):

• [x] https://github.com/snapframework/io-streams/issues/80