Add System.Text.Json to Directory.Build.props to fix MSB3277 (#1669)

The Contracts project targets netstandard2.0 and pulls System.Text.Json
9.0.x from NuGet. Every other project targets net8.0, which ships with
System.Text.Json 8.0.x in the framework. MSBuild sees both versions and
warns (MSB3277).

Adding an explicit System.Text.Json reference to Directory.Build.props
means all net8.0 projects pick up the 9.0.x NuGet package instead of
relying on the framework copy. One line, fixes the warning everywhere.

Author: JamieMagee

Directory.Build.props

@@ -32,6 +32,7 @@
     <PackageReference Include="Microsoft.VisualStudio.Threading.Analyzers" PrivateAssets="all"/>
     <PackageReference Include="MinVer" PrivateAssets="all"/>
     <PackageReference Include="StyleCop.Analyzers" PrivateAssets="all"/>
+    <PackageReference Include="System.Text.Json" />
   </ItemGroup>
 
 </Project>

src/Microsoft.ComponentDetection.Contracts/Microsoft.ComponentDetection.Contracts.csproj

@@ -14,7 +14,6 @@
         <PackageReference Include="packageurl-dotnet" />
         <PackageReference Include="System.Memory" />
         <PackageReference Include="System.Reactive" />
-        <PackageReference Include="System.Text.Json" />
         <PackageReference Include="System.Threading.Tasks.Dataflow" />
     </ItemGroup>