GHC 8.4 Support

[parsonsmatt]

In #1499, we had to drop GHC 8.4 support due to aeson and shakespeare constraints - the version of shakespeare that supports aeson >= 2 has a base >= 4.12 bound which rejects the base-4.11 used by GHC 8.4.

I filed a PR to support this in shakespeare, which should restore support, but the last LTS resolver with 8.4 was published 5 years ago. It may be time to let it go.

Supporting GHC 8.6 at minimum means we can go through and remove the CPP for supporting old things, particularly:

• base < 4.12
• template-haskell < 2.14

As well as enabling features easily that were introduced, like DerivingVia.