Enabler: Change github actions runner version

[arcepaul]

pa11y-ci borks in Actions when using ubuntu-22.04. Change workflow yaml and pin to ubuntu-20.04.

ESTIMATE 1

Related

• https://github.com/NCIOCPL/ncids/issues/671
• GitHub Actions: Ubuntu-latest workflows will use Ubuntu-22.04
• https://github.com/pa11y issue 651
• https://github.com/pa11y/pa11y-ci pull 197

[sarinapadilla]

Closing this ticket, as this was fixed with this ticket in cgov-react-app-playground!