Updates GitHub actions CI workflow to latest Node version

Also cleans up the workflow definition.

[x] Stay on point and keep it small so it can be easily reviewed. For example, try to apply any general refactoring separately outside of the PR.
~~[ ] Consider adding unit tests, especially for bug fixes. If you don't, tell us why.~~
[x] All new and existing tests pass, with 100% test coverage (npm run test:coverage)
[x] Linting passes (npm run lint)
~~[ ] Consider updating documentation. If you don't, tell us why.~~
[x] List the environments / platforms in which you tested your changes. GitHub Actions only

springload / draftail