Bump System.Text.Json in /UITests/UITests.Tests.MSTest

Bumps System.Text.Json from 5.0.2 to 8.0.4.

---
updated-dependencies:
- dependency-name: System.Text.Json
  dependency-type: direct:production
...

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

UITests/UITests.Tests.MSTest/UITests.Tests.MSTest.csproj

@@ -20,7 +20,7 @@
     <!-- Microsoft.UI.Xaml MSTest Extension -->
     <PackageReference Include="MUXTestInfra.MSTest" Version="0.0.4" />
     <!-- System packages -->
-    <PackageReference Include="System.Text.Json" Version="5.0.2" />
+    <PackageReference Include="System.Text.Json" Version="8.0.4" />
     <PackageReference Include="System.Drawing.Primitives" Version="4.3.0" />
     <PackageReference Include="System.Diagnostics.Process" Version="4.3.0" />
     <PackageReference Include="System.Reflection.TypeExtensions" Version="4.5.1" />