Use GitHub Actions for CI

[orien]

Summary

To give external development teams and the wider community confidence that this project is compatible with the latest versions of Ruby, I propose using GitHub Actions to automatically run the unit tests against a matrix of supported Ruby versions.

This GitHub workflow is passing on my fork and will be added to this project upon merge.

Checklist

• [x] Added changelog entry (If there isn't an #unreleased section, add that and your changelog entry to the top of the changelog)
• [x] Ran unit tests (rake test:unit)
• [x] I understand that unless this is a Draft PR or has a DO NOT MERGE label, this PR is considered to be in a deploy ready state and can be deployed if merged to main

[orien]

@hollabaq86 what do you think of this proposal?

[orien]

🍃