Run tests and examples against .NET 9 RC2

[bkoelman]

Demonstrates that all tests are green when running on .NET 9 RC2 with EF Core 9 RC2, while still referencing JADNC that targets only .NET 8 and 6.

While there are breaking changes in the EF Core driver API (so you'll need matching versions for PostgreSQL, MySQL, SQL Server, etc.), it appears that JADNC itself is already compatible. This means we don't need to release a new version of JADNC when .NET 9 ships.

QUALITY CHECKLIST

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

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 90.73%. Comparing base (55f671c) to head (c11c0c2). Report is 9 commits behind head on master.

Files with missing lines	Patch %	Lines
src/Examples/DapperExample/Program.cs	0.00%	2 Missing :warning:

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1619   +/-   ##
=======================================
  Coverage   90.73%   90.73%           
=======================================
  Files         353      353           
  Lines       11492    11492           
  Branches     1888     1888           
=======================================
  Hits        10427    10427           
  Misses        696      696           
  Partials      369      369           

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[bkoelman]

Superseded by #1656.