Drop support for .NET 6, test against .NET 9

[bkoelman]

Drop support for .NET 6, which is no longer being serviced since November 12, 2024.

Multi-target tests and examples against .NET 8 and 9.

Note

For using MySQL on .NET 9, there is no stable release yet.

QUALITY CHECKLIST

• Changes implemented in code
• Complies with our contributing guidelines
• Adapted tests
• N/A: Documentation updated

[codecov]

Codecov Report

Attention: Patch coverage is 99.35149% with 5 lines in your changes missing coverage. Please review.

Project coverage is 90.68%. Comparing base (c4dde57) to head (22507c6).
Report is 9 commits behind head on master.

Files with missing lines	Patch %	Lines
src/Examples/DapperExample/Program.cs	33.33%	2 Missing ⚠️
...c/JsonApiDotNetCore/Configuration/ResourceGraph.cs	66.66%	2 Missing ⚠️
...piDotNetCore/Diagnostics/AspNetCodeTimerSession.cs	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1656      +/-   ##
==========================================
+ Coverage   90.66%   90.68%   +0.01%     
==========================================
  Files         355      354       -1     
  Lines       11559    11535      -24     
  Branches     1905     1900       -5     
==========================================
- Hits        10480    10460      -20     
+ Misses        705      701       -4     
  Partials      374      374              

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