Fix CI warnings and update GitHub action versions

[kurtmckee]

This PR introduces the following changes:

• Eliminate CI warnings caused by use of the set-output, which is deprecated. Recent example:

  

• Migrate to simply using the python-version output of the setup-python action.

  This fixes a bug that caused Windows caches to all be shared for lack of a Python version in the cache key. Recent example filename:

  venv-pull_request-Windows--1621fe5fd7c3b6ecee2af3dd004805948e7c035d688b98337eac07026264ed96
  

• Update all GitHub actions to their latest versions.

  Dependabot doesn't appear to be submitting PRs to keep GitHub actions updated (it's only submitting Python- and Docker-related PRs). This may be a setting at the org level that needs to be changed, or perhaps a setting for the individual repository.

Taken together, these changes should help ensure CI runs without warnings. The Dependabot updates need to be investigated by someone with org/repo permissions, though.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 92.09%. Comparing base (49109f7) to head (97e44a9). Report is 13 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #414   +/-   ##
=======================================
  Coverage   92.09%   92.09%           
=======================================
  Files          24       24           
  Lines         759      759           
  Branches       80       80           
=======================================
  Hits          699      699           
  Misses         42       42           
  Partials       18       18           

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