Fix FSharp.Core downgrade warning

Author: MangelMaxime

src/FSharpLanguageServer/FSharpLanguageServer.fsproj

@@ -21,6 +21,7 @@
   <ItemGroup>
     <PackageReference Include="FSharp.Compiler.Service" Version="39.0.0" />
     <PackageReference Include="HtmlAgilityPack" Version="1.8.4" />
+    <PackageReference Update="FSharp.Core" Version="5.0.1" />
   </ItemGroup>
 
   <ItemGroup>