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[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

```diff @@ 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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