Bump System.Text.Json in /Mono.TextTemplating.Build.Tests

Bumps System.Text.Json from 7.0.3 to 8.0.4.

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

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

Mono.TextTemplating.Build.Tests/Mono.TextTemplating.Build.Tests.csproj

@@ -21,7 +21,7 @@
     and loads first, thereby breaking loading of MSBuild assemblies. Force-upgrade it.
     -->
     <PackageReference Include="NuGet.Frameworks" Version="6.9.1" />
-    <PackageReference Include="System.Text.Json" Version="7.0.3" />
+    <PackageReference Include="System.Text.Json" Version="8.0.4" />
   </ItemGroup>
 
   <ItemGroup>