Do not reference System.Text.Json for .NET 8+ (#4019)

#3933 mistakenly removed
the target framework condition for STJ. Referencing the STJ package on
.NET 8+ causes the STJ source generator to run twice, resulting in build
errors. See dotnet/wpf#8943.

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**What kind of change does this PR introduce?**
<!-- Bug fix, feature, docs update, ... -->

Bug fix

**What is the current behavior?**
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Build errors for projects referencing ReactiveUI and using STJ source
generation at the same time.

**What is the new behavior?**
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No build errors.

**What might this PR break?**



**Please check if the PR fulfills these requirements**
- [ ] Tests for the changes have been added (for bug fixes / features)
- [ ] Docs have been added / updated (for bug fixes / features)

**Other information**:

Author: database64128

src/ReactiveUI/ReactiveUI.csproj

@@ -58,10 +58,12 @@
     <Compile Include="Platforms\net\**\*.cs" />
     <PackageReference Include="System.ComponentModel.Annotations" />
   </ItemGroup>
+  <ItemGroup Condition="$(TargetFramework.StartsWith('netstandard')) or $(TargetFramework.StartsWith('net4'))">
+    <PackageReference Include="System.Text.Json" />
+  </ItemGroup>
   <ItemGroup>
     <PackageReference Include="Splat" />
     <PackageReference Include="DynamicData" />
-    <PackageReference Include="System.Text.Json" />
   </ItemGroup>
   <ItemGroup>
     <None Update="VariadicTemplates.tt" Generator="TextTemplatingFileGenerator" LastGenOutput="VariadicTemplates.cs" />