Bump System.Reactive from 5.0.0 to 6.0.0

Bumps [System.Reactive](https://github.com/dotnet/reactive) from 5.0.0 to 6.0.0.
- [Release notes](https://github.com/dotnet/reactive/releases)
- [Commits](https://github.com/dotnet/reactive/commits)

---
updated-dependencies:
- dependency-name: System.Reactive
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

Refit.Tests/Refit.Tests.csproj

@@ -17,7 +17,7 @@
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.5.0" />
     <PackageReference Include="xunit" Version="2.4.2" />
     <PackageReference Include="xunit.runner.visualstudio" Version="2.4.5" />
-    <PackageReference Include="System.Reactive" Version="5.0.0" />    
+    <PackageReference Include="System.Reactive" Version="6.0.0" />    
     <PackageReference Include="Microsoft.AspNetCore.WebUtilities" Version="2.2.0" />
     <PackageReference Include="Microsoft.CodeAnalysis.CSharp.Workspaces" Version="4.0.1" />
     <PackageReference Include="Microsoft.CodeAnalysis.CSharp.SourceGenerators.Testing.XUnit" Version="1.1.2-beta1.23251.2" />