Add .NET 9 to target frameworks

[danielcweber]

No description provided.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.07%. Comparing base (b3d39e0) to head (33bc9d3).
Report is 6 commits behind head on 12.x.

Additional details and impacted files

@@           Coverage Diff           @@
##             12.x    #1799   +/-   ##
=======================================
  Coverage   83.07%   83.07%           
=======================================
  Files         255      255           
  Lines        6891     6891           
  Branches     1507     1507           
=======================================
  Hits         5725     5725           
  Misses        514      514           
  Partials      652      652           

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