horizon: Completely migrate CI from Circle to GitHub actions

[2opremio]

We are already building and testing the monorepo with GitHub actions (see https://github.com/stellar/go/blob/master/.github/workflows/go.yml ). However, there are a few things left:

• Integration tests (with and without captive core).
• Build, test and push the verify-range Docker image
• When releasing:
  • Build the release artifacts and upload them to the release
  • Publish the horizon docker image in Dockerhub (which is put on hold until manually approved) (we will relay on Jenkins for this, which pushes stellar/stellar-horizon:{latest,<version>} when stabilizing a debian package).
• Publish the horizon docker image to DockerHub for every commit in master
• Publish the state-diff Docker image to DockerHub
• Change the repo configuration to use the GitHub actions as a mandatory successful check
• Remove CircleCI configuration
• Fully enable release jobs and watch them closely during a release