Improve GitHub actions configuration

[orangejulius]

The schema repository is a little different than our other repositories, since it runs quite a few more integration tests and has more external dependencies (JVM, Elasticsearch).

We could probably eventually unify it's GitHub Actions config with all our other repositories, but for now this mostly updates the existing config with some new best practices:

• Run all tests against Node.js 12, 14, and 16
• Update jobs to run on Ubuntu 20 (Ubuntu 16 is deprecated: https://github.blog/changelog/2021-04-29-github-actions-ubuntu-16-04-lts-virtual-environment-will-be-removed-on-september-20-2021/)
• Run docker build after any NPM publish, so that versioned docker images can be built
• Remove any CircleCI config that exists (we can use Github Actions to build docker images just fine)
• Remove any lingering references to TravisCI