Allow roll-forward for dotnet-validate

[martincostello]

Fix package validation by allowing dotnet-validate to roll-forward to a newer .NET runtime than .NET 6.

Otherwise when the GitHub Actions ubuntu-latest label rolls over to Ubuntu 24.04, .NET 6 will no longer be installed and the job will fail. See actions/runner-images#10636.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.39%. Comparing base (e8387fb) to head (a8e8742).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2418      +/-   ##
==========================================
+ Coverage   85.36%   85.39%   +0.02%     
==========================================
  Files         312      312              
  Lines        7464     7464              
  Branches     1121     1121              
==========================================
+ Hits         6372     6374       +2     
+ Misses        906      905       -1     
+ Partials      186      185       -1     

Flag	Coverage Δ
linux	85.36% <ø> (ø)
macos	85.36% <ø> (ø)
windows	85.36% <ø> (+0.02%)	⬆️

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