Upgrade SDK to .NET 9

[VMelnalksnis]

Fixes # .

Changes proposed in this pull request:
*

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 6 lines in your changes missing coverage. Please review.

Project coverage is 90.39%. Comparing base (861681a) to head (fd670b7).
Report is 2 commits behind head on master.

✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...DependencyInjection/ServiceCollectionExtensions.cs	16.66%	5 Missing ⚠️
...lksnis.PaperlessDotNet/Documents/DocumentClient.cs	88.88%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #275      +/-   ##
==========================================
- Coverage   91.30%   90.39%   -0.91%     
==========================================
  Files          32       32              
  Lines         529      531       +2     
  Branches       55       55              
==========================================
- Hits          483      480       -3     
- Misses         31       36       +5     
  Partials       15       15              

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[cocogitto-bot]

✔️ 7f32649...fd670b7 - Conventional commits check succeeded.