Use declarative version specification for Ubuntu GitHub Actions runners

[d33bs]

Description

This PR addresses #197 by setting a specific version for Ubuntu runners through GitHub Actions. It follows thoughts from #195 where we experienced surprising issues after macos-latest began referencing versions which aren't compatible with Python versions we remain compatible with here.

Thank you for any thoughts or feedback you may have!

Closes #197

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• Bug fix (fixes an issue).
• Enhancement (adds functionality).
• Breaking change (fix or feature that would cause existing functionality to not work as expected).
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[d33bs]

Thanks @kenibrewer ! Merging this in.