Re-enable with-utf8 and th-env

[kirelagin]

• The latest version of with-utf8 works with GHC up to and including 9.2.1.
• The latest version of th-env has been fixed to work with GHC 9.

Checklist:

• Meaningful commit message, eg add my-cool-package (please don't mention build-constraints.yml)

• At least 30 minutes have passed since uploading to Hackage

• On your own machine, you have successfully run the following command (find verify-package in the root of this repo):

  ./verify-package $package # or $package-$version
  

The script runs virtually the following commands in a clean directory:

  stack unpack $package-$version # `-$version` is optional
  cd $package-$version
  rm -f stack.yaml && stack init --resolver nightly --ignore-subdirs
  stack build --resolver nightly --haddock --test --bench --no-run-benchmarks

CI

Our CI tries to line up build-constraints.yaml with the current state
of Hackage. This means that failures that are unrelated to your PR may
cause the check to fail. If you think a failure is unrelated you can
simply ignore it and the Curators will let you know if there is
anything you need to do.

[kirelagin]

The check failure seems to be unrelated. stack test passes with current nightly + th-env-0.1.0.3 + happy-1.20.0 (#6252).

[DanBurton]

thanks!