Bump GHC version for LTS 16.12
Bump GHC version for LTS 16.12

Fixes #5642

fpco/stackage,jonschoning/stackage,clinty/stackage,michalkonecny/stackage,iand675/stackage,soenkehahn/stackage,massysett/stackage,simonmichael/stackage,Daniel-Diaz/stackage,mrkkrp/stackage