.NET 10 preparation

[martincostello]

Cherrypick some changes from #2531 while that is blocked on a new release of Stryker that supports .NET 10 GA.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.15%. Comparing base (aa6f02c) to head (0923fae).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2842   +/-   ##
=======================================
  Coverage   96.15%   96.15%           
=======================================
  Files         309      309           
  Lines        7123     7127    +4     
  Branches     1005     1005           
=======================================
+ Hits         6849     6853    +4     
  Misses        221      221           
  Partials       53       53           

Flag	Coverage Δ
linux	96.15% <100.00%> (+<0.01%)	⬆️
macos	96.15% <100.00%> (+<0.01%)	⬆️
windows	96.14% <100.00%> (+<0.01%)	⬆️

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[Copilot]

Pull request overview

This PR prepares the codebase for .NET 10 by cherrypicking selected changes from PR #2531. The changes focus on updating target framework prioritization, adding .NET 9.0 SDK support to CI workflows, updating dependencies, and refactoring internal field usage in bulkhead policies.

• Reordered target frameworks to prioritize .NET 9.0 over .NET 8.0 in project files
• Added .NET 9.0.x SDK to GitHub Actions workflows for CI/CD
• Refactored bulkhead policy classes to use auto-properties instead of private fields
• Updated Newtonsoft.Json dependency version

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
test/Polly.Specs/Polly.Specs.csproj	Reordered target frameworks to prioritize .NET 9.0
src/Polly/Bulkhead/BulkheadPolicy.cs	Refactored _maxQueueingActions field to MaxQueueingActions property in both generic and non-generic classes
src/Polly/Bulkhead/AsyncBulkheadPolicy.cs	Refactored _maxQueueingActions field to MaxQueueingActions property in both generic and non-generic classes
bench/Polly.Benchmarks/Polly.Benchmarks.csproj	Reordered target frameworks to prioritize .NET 9.0
Directory.Packages.props	Updated Newtonsoft.Json version from 13.0.3 to 13.0.4
.github/workflows/mutation-tests.yml	Added .NET 9.0.x SDK setup for mutation testing
.github/workflows/build.yml	Added .NET 9.0.x SDK setup for main build workflow

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