Upgrade to dotnet v8

[kayoub5]

• .NET 6 Supports ends 12 Nov 2024, and both .NET 7 and 8 have been released.
• Changing target framework of the examples only, the main library still target .NET Standard
• .NET 8 is not available on Ubuntu 18.04, Ubuntu 18.04 already reached end of life (only paid Extended Security Maintenance is available), so it would make sense to just remove Ubuntu from AppVeyor

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (dbf3b58) 82.71% compared to head (c5bd3f0) 83.13%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #495      +/-   ##
==========================================
+ Coverage   82.71%   83.13%   +0.42%     
==========================================
  Files          51       51              
  Lines        2823     2823              
  Branches      312      312              
==========================================
+ Hits         2335     2347      +12     
+ Misses        365      356       -9     
+ Partials      123      120       -3     

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