Update dependency System.Reactive to v6

Author: renovate[bot]

Directory.Packages.props

@@ -47,7 +47,7 @@
     <PackageVersion Include="XunitXml.TestLogger" Version="3.0.78" />
     <PackageVersion Include="coverlet.collector" Version="3.2.0" />
     <PackageVersion Include="coverlet.msbuild" Version="3.2.0" />
-    <PackageVersion Include="System.Reactive" Version="5.0.0" />
+    <PackageVersion Include="System.Reactive" Version="6.0.0" />
     <PackageVersion Include="System.Collections.Immutable" Version="5.0.0" />
     <PackageVersion Include="System.Threading.Channels" Version="6.0.0" />
     <PackageVersion Include="Microsoft.Reactive.Testing" Version="5.0.0" />