stack: Bump default resolver to 18.14
stack: Bump default resolver to 18.14

No impact on CI, just makes it easier for contributors to use the latest
8.10 GHC.

Related: 48f7c47821662b489dd8337f8e0c2be9354fac4f@f5f6ef41cb6cce3ba14957c31640f10b5751c90c

xmonad/xmonad