swaps from default nbclassic and opt-in to lab, to now default to lab and opt-in to nbclassic
defaults to jupyterlab *if* lab 3.1 is available,
so should still work without configuration if lab is unavailable (or too old)
- update references to default branch name in docs, workflows
- use HEAD in github urls, which always works regardless of default branch name
- fix petstore URLs since the old petstore links seem to have stopped working
