Update gh actions checkout version

[dschmitz89]

Currently many CI jobs show the following deprecation warning (see for example here):

This PR updates the "checkout" gh action to the latest major release.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.20%. Comparing base (625d767) to head (727a077).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1386      +/-   ##
===========================================
- Coverage    95.35%   95.20%   -0.16%     
===========================================
  Files          825      616     -209     
  Lines        68203    54107   -14096     
===========================================
- Hits         65033    51510   -13523     
+ Misses        3170     2597     -573     

see 236 files with indirect coverage changes

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