Raise scotty-0.20's lower version bounds on base

[RyanGlScott]

As discovered in this CI job, cabal can pick a build plan with scotty-0.20 and GHC 8.4, even though 8.4 doesn't support building scotty-0.20:

Failed to build scotty-0.20.
Build log ( /github/home/.cabal/logs/ghc-8.4.4/scotty-0.20-6d4ed606ddf8cc6e1fb58b8a4a4553c26e357584de0804cc06b6bd01dcc3e932.log ):
Configuring library for scotty-0.20..
Preprocessing library for scotty-0.20..
Building library for scotty-0.20..

Web/Scotty/Internal/Types.hs:5:14: error:
    Unsupported extension: DerivingVia
  |
5 | {-# language DerivingVia #-}
  |              ^^^^^^^^^^^

Can you raise scotty-0.20's lower version bounds on base to >=4.12 to reflect this? Actually, it might even be worth raising it to >=4.14, since this PR indicates that only GHC 8.10 and later are officially supported.

[ocramz]

Ah yes, I added DerivingVia because I wanted to autoderive the Alternative instance of ActionT via Ap but I ended up writing the instance by hand. Sorry!

[ocramz]

I've made a revision to scotty-0.20, now base >= 4.14 .