FIx build errors with newer toolset and dependencies

Fixes the following two errors:
- src/Features/Core/Portable/ExternalAccess/Watch/Api/WatchHotReloadService.cs(217,13): error CS1503: Argument 4: cannot convert from 'System.Collections.Immutable.ImmutableHashSet<Microsoft.CodeAnalysis.Project>' to 'System.Collections.Generic.IReadOnlySet<Microsoft.CodeAnalysis.Project>' [src/Features/Core/Portable/Microsoft.CodeAnalysis.Features.csproj::TargetFramework=net9.0]
- src/roslyn/src/Features/Core/Portable/ExternalAccess/Watch/Api/WatchHotReloadService.cs(218,13): error CS1503: Argument 5: cannot convert from 'System.Collections.Immutable.ImmutableHashSet<Microsoft.CodeAnalysis.Project>' to 'System.Collections.Generic.IReadOnlySet<Microsoft.CodeAnalysis.Project>' [src/roslyn/src/Features/Core/Portable/Microsoft.CodeAnalysis.Features.csproj::TargetFramework=net9.0]

The underlying SDK being used is "10.0.100-alpha.1.24555.54"

Author: ViktorHofer

src/Features/Core/Portable/Microsoft.CodeAnalysis.Features.csproj

@@ -139,7 +139,7 @@
     <PackageReference Include="Microsoft.CodeAnalysis.AnalyzerUtilities" />
     <PackageReference Include="Microsoft.CodeAnalysis.Elfie" Condition="'$(DotNetBuildSourceOnly)' != 'true'" />
     <PackageReference Include="Microsoft.DiaSymReader" />
-    <PackageReference Include="System.Text.Json" />
+    <PackageReference Include="System.Text.Json" Condition="'$(TargetFrameworkIdentifier)' != '.NETCoreApp'" />
     <PackageReference Include="System.Threading.Tasks.Extensions" Condition="'$(TargetFrameworkIdentifier)' != '.NETCoreApp'" />
   </ItemGroup>
   <Import Project="..\..\..\Analyzers\Core\Analyzers\Analyzers.projitems" Label="Shared" />