Migrate to Actions v2

[smockle]

Migrates from GitHub Actions v1 to Actions v2.

Rationale:

• The documentation now presupposes Actions v2 usage
• The “Workflows defined in .github/workflows” link in the “Actions” tab is currently broken (screenshot below):

Changes:

• Move workflow file into the .github/workflows directory
• Replace HCL with YAML
• Remove unneeded uses = "actions/npm@v2.0.0" (see the Node.js sample workflow)

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[smockle]

To eliminate future merge conflicts, I’d originally attempted to incorporate the changes in #450 into this PR. But I was stumped by 401s during attempts to publish to GPR, e.g. https://github.com/primer/components/commit/409e586758051b62243b677ed7d715ccabf52236/checks. I spent some time debugging, then decided to keep this PR small and focused on the migration to Actions v2.

[emplums]

Just left one request for changes! Thank you for doing this, I've been meaning to get around to it! 🎉

[emplums]

I'm not super worried about keeping the publish to GPR stuff - we can circle back on that later and re-set it up! It didn't take too long and that PR is pretty stale so we might as well start fresh :)

[emplums]

👍 🎉