Set ghc-8.6.3 as the recommended version

It seems to work well with current ecosystem and stackage has just introduced 8.6.3 as part of lts-13.1: https://www.stackage.org/lts-13.1

Cherry-picked at https://github.com/haskell/ghcup/commit/e0b1e5afeffdbb0a7bb937698563ccdc1e47edd8