In workflow, update default branch to main, use ubuntu-20.04

[danyalaytekin]

Warning

Instamerge to follow with forgiveness requested.
But reviewed elsewhere for other repos, keeping it minimal here, and workflow is running again/passing.

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Minimal workflow updates to bring build closer to working again:

• workflow trigger: use main in place of master for default branch name
• anticipate build error: use ubuntu-20.04 in place of ubuntu-latest, to match its use in pa11y-webservice until pa11y@7 is released