Add the GHC used by Hackage to the build matrix

[sellout]

We should try to ensure that our build will behave properly on Hackage (which it currently doesn’t seem to do).

This also simplifies the tested-with field.

[sellout]

So, if this isn’t failing, it’s not worth merging, because it’s not modeling Hackage closely enough.

Alternatively, if we upload the docs to Hackage ourselves (which is probably better than realizing they’re broken after uploading to Hackage), then Hackage probably doesn’t bother to build the package at all, and we don’t need this PR at all.