Rename the 8 default branches to `main` from `master`

[danyalaytekin]

Taking care with GitHub Actions workflows, which don't appear to forward master to main, or to offer a variable (one which will work in on) to represent the default branch.

• [x] pa11y/org, renamed Oct 6th, 2023
• [x] pa11y/pa11y.github.io, renamed Oct 4th, 2023
• [x] pa11y/pa11y, renamed Oct 9th, 2023
• [x] pa11y/pa11y-ci, renamed Oct 26th, 2023
• [x] pa11y/pa11y-dashboard, renamed Oct 26th, 2023
• [x] pa11y/pa11y-lint-config, renamed Oct 7th, 2023
• [x] pa11y/pa11y-webservice, renamed Oct 26th, 2023
• [x] pa11y/pa11y-webservice-client-node, renamed Oct 6th, 2023

More about main: https://github.com/github/renaming