Switch from Circle CI to GitHub Actions

The testing docker image has been split up. Instead of having one image
with all supported versions of zsh installed, we now have a separate
image for each supported zsh version.

We use GitHub Action matrices to run jobs in parallel for all of the
supported versions.

We no longer need to publish images to Docker Hub. The images are just
built by CI (or developers) as needed from the Dockerfile in the repo.

ZSH_VERSIONS

@@ -1,9 +1,5 @@
 # Zsh releases to run tests against
 # See https://github.com/zsh-users/zsh/releases
-#
-# When modifying this file, rebuild and push docker image:
-#   $ docker build -t ericfreese/zsh-autosuggestions-test .
-#   $ docker push ericfreese/zsh-autosuggestions-test
 4.3.11
 5.0.2
 5.0.8