Pin version of FSharp.Core

Otherwise the version is determined by the SDK, which can conflict with <RestoreLockedMode>

dotnet/fsharp#12315

Author: jnyrup

Tests/FSharp.Specs/FSharp.Specs.fsproj

@@ -3,13 +3,15 @@
   <PropertyGroup>
     <TargetFramework>net6.0</TargetFramework>
     <LangVersion>6.0</LangVersion>
+    <DisableImplicitFSharpCoreReference>true</DisableImplicitFSharpCoreReference>
   </PropertyGroup>
 
   <ItemGroup>
     <Compile Include="FSharpSpecs.fs" />
   </ItemGroup>
 
   <ItemGroup>
+    <PackageReference Include="FSharp.Core" Version="9.0.303" />
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.13.0" />
     <PackageReference Include="xunit" Version="2.9.3" />
     <PackageReference Include="xunit.runner.visualstudio" Version="3.0.0">