Bump Microsoft.NETCore.UniversalWindowsPlatform in /Rx.NET/Source

Bumps [Microsoft.NETCore.UniversalWindowsPlatform](https://github.com/Microsoft/dotnet) from 6.1.9 to 6.2.8.
- [Release notes](https://github.com/Microsoft/dotnet/releases)
- [Commits](https://github.com/Microsoft/dotnet/commits)

Signed-off-by: dependabot-preview[bot] <[email protected]>

Rx.NET/Source/tests/Tests.System.Reactive.Uwp.DeviceRunner/Tests.System.Reactive.Uwp.DeviceRunner.csproj

@@ -90,7 +90,7 @@
     <UseDotNetNativeToolchain>true</UseDotNetNativeToolchain>
   </PropertyGroup>
   <ItemGroup>
-    <PackageReference Include="Microsoft.NETCore.UniversalWindowsPlatform" Version="6.1.9" />
+    <PackageReference Include="Microsoft.NETCore.UniversalWindowsPlatform" Version="6.2.8" />
     <PackageReference Include="xunit" Version="2.4.1" />
     <PackageReference Include="xunit.runner.devices" Version="2.5.25" />
   </ItemGroup>