Add .NET 9 to target frameworks

[sveinungf]

No description provided.

[github-actions]

Test Results

     9 files  ±0       9 suites  ±0   16m 13s ⏱️ + 3m 33s
 4 549 tests ±0   4 549 ✅ ±0  0 💤 ±0  0 ❌ ±0 
13 341 runs  ±0  13 341 ✅ ±0  0 💤 ±0  0 ❌ ±0 

Results for commit 44e98eb. ± Comparison against base commit 367e5c5.

[codecov]

Codecov Report

Attention: Patch coverage is 40.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 91.92%. Comparing base (367e5c5) to head (44e98eb).
Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
SpreadCheetah/SpreadsheetBuffer.cs	40.00%	0 Missing and 3 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #96   +/-   ##
=======================================
  Coverage   91.91%   91.92%           
=======================================
  Files         152      152           
  Lines        5579     5584    +5     
  Branches      929      934    +5     
=======================================
+ Hits         5128     5133    +5     
  Misses        213      213           
  Partials      238      238           

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