Directory.Packages.props,FX.Tests: move version attrib out of proj

In order to fix this restore error:
```
/__w/FX/FX/src/FX.Tests/FX.Tests.csproj : error NU1008: Projects that use
central package version management should not define the version on the
PackageReference items but on the PackageVersion items: FSharp.Core.
[/__w/FX/FX/FX.sln]
```

Directory.Packages.props

@@ -13,5 +13,6 @@
     <PackageVersion Include="Giraffe.Razor" Version="1.3.0" />
     <PackageVersion Include="Microsoft.AspNetCore.WebSockets" Version="2.1.1" />
     <PackageVersion Include="TaskBuilder.fs" Version="2.1.0" />
+    <PackageVersion Include="FSharp.Core" Version="8.0.101" />
   </ItemGroup>
 </Project>

src/FX.Tests/FX.Tests.csproj

@@ -10,7 +10,7 @@
     <PackageReference Include="Microsoft.NET.Test.Sdk" />
     <PackageReference Include="StackExchange.Redis" />
     <PackageReference Include="Newtonsoft.Json" />
-    <PackageReference Include="FSharp.Core" Version="8.0.101" />
+    <PackageReference Include="FSharp.Core" />
   </ItemGroup>
   <ItemGroup>
     <Compile Remove="Class1.cs" />