Update VERSION to 2.1.0

[sumobrian]

This PR updates the VERSION file to 2.1.0.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.95%. Comparing base (04d4d43) to head (86ea799).
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #1167   +/-   ##
=========================================
  Coverage     80.95%   80.95%           
  Complexity     2996     2996           
=========================================
  Files           407      407           
  Lines         15229    15229           
  Branches       1023     1023           
=========================================
  Hits          12328    12328           
  Misses         2272     2272           
  Partials        629      629           

Flag	Coverage Δ
unittests	80.95% <ø> (ø)

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