Use conformance runner binary for conformance tests

[smaye81]

This performs a few updates to the repository:

• Switches from CircleCI to Github Actions for CI
• Updates to Node 16 for the CI environment
• Updates to use the published conformance runner binaries in Buf's protobuf-conformance repo.

[timostamm]

This is awesome, thanks Steve!

We are using protoc v3.12.0, and the corresponding proto files in packages/test-fixtures, but v22.3 of the conformance test runner. We have to keep them in sync. Let me take a closer look, maybe we can update in a follow-up.

[smaye81]

Yeah, I considered that and was actually thinking a follow-up would be better if possible. I have a feeling the follow-up is going to be a decent amount of changes and didn't want to clutter this PR if we didn't have to. But, if it's better to stay in sync, I'm cool with doing it here.

[timostamm]

Let's do it one step at a time, LGTM.