Update generic-lens version bound

[aherrmann]

To address commercialhaskell/stackage#7828

[aherrmann]

tested locally with

$ cabal update
$ cabal v2-configure --constraint=generic-lens==2.3.0.0
$ cabal v2-build
$ cabal v2-test

I'll create a new Hackage revision as well.

[aspiwack]

I'm going to do my bit again, but does this really need an upper bound? Does any of this?

Anyway…

[aspiwack]

Thanks for being prompt, and sorry for the delay before reviewing.