Updates action versions, change how notebooks are tested

[billbrod]

This does two things:

• Updates action versions to get rid of the warning during github actions
• Changes how notebooks are tested: we were previously parallelizing across all of them, which I think was actually costing us time (and making it output to navigate), so now we run them all in one action.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.76%. Comparing base (1ac604b) to head (3cc0755).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #257   +/-   ##
=======================================
  Coverage   86.76%   86.76%           
=======================================
  Files          35       35           
  Lines        3325     3325           
=======================================
  Hits         2885     2885           
  Misses        440      440           

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[billbrod]

Turns out that this now looks like it will 22 to 28 minutes for the notebooks, whereas before, if things ran successfully, they would take ~20 minutes total. I think the small increase in time is worth it, given that our previous setup would occasionally just hang during one of the notebooks, and how difficult it was to navigate the output.