dhall: BREAKING CHANGE update dhall config for 1.33+ compatibility

dhall: BREAKING CHANGE update dhall config for 1.33+ compatibility

Since dhall 1.33.0 lang version the `Optional/fold` function is
removed from the language and can be replaced by the usage of the
`merge` function which now works on Optional values 🎉 since 1.29

https://github.com/dhall-lang/dhall-haskell/pull/1608

This new code is thus not compatible with versions of dhall
<1.29.0 (due to the new feature of the `merge` fonction).