Update CI pipeline steps

[jmert]

• Update checkout and code coverage actions version
• Add version downgrade step for backwards-compatibility testing (which has no effect on this repository as it has no external dependencies, but it's being included for consistency with other CI definitions).
• Minor syntax tweaks (again, for consistency)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (b08d1e0) to head (50a86f8).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master       #18   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            1         1           
  Lines          190       190           
=========================================
  Hits           190       190           

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