Refactor wasmappbuilder's System.Text.Json reference to use a versions.props property (#70582)

* Refactor wasmappbuilder's System.Text.Json version reference to use a versions.props property

* Update per code review and refactor MetadataLoadContext version

Author: MichaelSimons

eng/Versions.props

@@ -100,6 +100,7 @@
     <SystemMemoryVersion>4.5.4</SystemMemoryVersion>
     <SystemNumericsVectorsVersion>4.5.0</SystemNumericsVectorsVersion>
     <SystemReflectionMetadataVersion>6.0.0</SystemReflectionMetadataVersion>
+    <SystemReflectionMetadataLoadContextVersion>4.7.1</SystemReflectionMetadataLoadContextVersion>
     <SystemReflectionEmitVersion>4.7.0</SystemReflectionEmitVersion>
     <SystemReflectionEmitILGenerationVersion>4.7.0</SystemReflectionEmitILGenerationVersion>
     <SystemReflectionEmitLightweightVersion>4.7.0</SystemReflectionEmitLightweightVersion>

src/tasks/WasmAppBuilder/WasmAppBuilder.csproj

@@ -20,8 +20,12 @@
 
     <PackageReference Include="Microsoft.Build" Version="$(MicrosoftBuildVersion)" />
     <PackageReference Include="Microsoft.Build.Tasks.Core" Version="$(MicrosoftBuildTasksCoreVersion)" />
-    <PackageReference Include="System.Reflection.MetadataLoadContext" Version="4.7.1" />
-    <PackageReference Include="System.Text.Json" Version="6.0.0" PrivateAssets="all" />
+    <PackageReference Include="System.Reflection.MetadataLoadContext" Version="$(SystemReflectionMetadataLoadContextVersion)" />
+
+    <!-- When we requested a version that is newer than what comes with Visual Studio version of msbuild, it causes problems with
+         running the task from inside VS. Therefore, we are using the moving version only for source-build. -->
+    <PackageReference Include="System.Text.Json" Condition="'$(DotNetBuildFromSource)' != 'true'" Version="6.0.0" PrivateAssets="all" />
+    <PackageReference Include="System.Text.Json" Condition="'$(DotNetBuildFromSource)' == 'true'" Version="$(SystemTextJsonVersion)" PrivateAssets="all" />
   </ItemGroup>
 
   <Target Name="GetFilesToPackage" Returns="@(FilesToPackage)">