CONTRIBUTING: Correct default branch name and latest version

[mnoorenberghe]

Trivial doc changes. Perhaps it would be better to remove the number of the latest version so this won't become stale again?

[codecov]

Codecov Report

Merging #4339 (c2cf695) into main (7e1a70f) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##               main    #4339   +/-   ##
=========================================
  Coverage     31.84%   31.84%           
  Complexity    11099    11099           
=========================================
  Files          1091     1091           
  Lines         39145    39145           
=========================================
  Hits          12466    12466           
  Misses        26679    26679           

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