Fix GitHub Actions build

[danyalaytekin]

This pull request mainly fixes the build and introduces me to Pa11y PR creation.

1. Fixes the make test step of the GitHub Actions check checkout_and_test which had stopped working recently because of the Ubuntu issue currently affecting pa11y-ci. It does this by regressing the image from ubuntu-latest to ubuntu-20.04. Soon ubuntu-latest will return.
2. Makes several other tiny changes, some of which emerged primarily to bump the build, but which hopefully remain useful:
   1. Recommend a VS Code extension for make
   2. Add .nvmrc and set to same Node.js version as the CI check
   3. Make one instance of the copyright year current; republishing should fix the other instance
   4. Improve consistency where we mention GitHub and GitHub Issues

If the PR is approved I also plan to rename master to main a moment before merging it. This is part of verifying this renaming process before extending it to repos which have more consumers, and in this repo's case it will also republish the site, immediately after it is unpublished by the change of name.

[jpw]

LGTM! Thank you.

[danyalaytekin]

Renaming master → main now for:

• Rename the 8 default branches to main from master org#19

[danyalaytekin]

Renaming took GitHub about a minute and has not actually unpublished https://pa11y.org (yet?). Let's see whether the workflow's mention of master will map to main after the merge. There doesn't seem to be a variable like {{ github.default_branch }} that would work in the on: section of a workflow.

Update: renaming the default branch triggered its own republish.

GitHub's advisories before renaming

GitHub's notice once in progress

GitHub's notice once complete

[danyalaytekin]

Let's see whether the workflow's mention of master will map to main after the merge

It hasn't, so build_and_test hasn't run yet on main. Will create another PR for this, and it's good to know about for the other repos. We can use a temporary on: push: branches: [master, main].