Migrate from buildkite to github actions

[mandyschen]

What changed?
Removed uses of buildkite and used github actions instead.

Why?
We want to migrate away from using buildkite.

How did you test it?
I test using the Github Actions functionality in the UI.

Potential risks
Doesn't catch certain failures, flaky tests

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.13%. Comparing base (2af19f2) to head (1716229).
⚠️ Report is 1 commits behind head on master.
see 2 files with indirect coverage changes

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[mandyschen]

GRPC buildkite:

GRPC GA:

Sticky on buildkite:

Sticky on GA:

Sticky off buildkite:

Sticky off GA:

Unit test buildkite:

Unit test GA:

GA Artifacts:

Unit test has multiple: