Migrate from Azure Pipelines to GitHub Actions (#13222)

Beyond just a pure migration, this also:

- Drops Node 14 usage from CI (which also removes the need for explicit
  npm 8 installs)
- Removes the "old node" test, which just checked that installing on old
  node failed. We shouldn't need to test that
- Consolidates the build & test workflows so we don't end up with
  a proliferation of workflows.