Explicitly reference FSharp lang 9 in TypeLevel project

Author: gusty

src/FSharpPlus.TypeLevel/FSharpPlus.TypeLevel.fsproj

@@ -5,7 +5,7 @@
     <DefineConstants>$(DefineConstants);TYPELEVEL_DEBUG</DefineConstants>
     <Configurations>Debug;Release;Fable</Configurations>
     <Platforms>AnyCPU</Platforms>
-    <LangVersion Condition=" '$(Configuration)' == 'Fable'">5.0</LangVersion>
+    <LangVersion>9.0</LangVersion>
     <DefineConstants Condition=" '$(Configuration)' == 'Fable'">$(DefineConstants);FABLE_COMPILER</DefineConstants>
   </PropertyGroup>