Use langversion = 6

In order to avoid dotnet/fsharp#14313

Author: gusty

src/FSharpPlus/FSharpPlus.fsproj

@@ -21,7 +21,7 @@
     <AutoGenerateBindingRedirects>true</AutoGenerateBindingRedirects>
     <Configurations>Debug;Release;Fable;Fable3</Configurations>
     <Platforms>AnyCPU</Platforms>
-    <LangVersion Condition=" '$(Configuration)' == 'Fable' OR '$(Configuration)' == 'Fable3' ">6.0</LangVersion>
+    <LangVersion>6.0</LangVersion>
     <DefineConstants Condition=" '$(Configuration)' == 'Fable'">$(DefineConstants);FABLE_COMPILER</DefineConstants>
     <DefineConstants Condition=" '$(Configuration)' == 'Fable3'">$(DefineConstants);FABLE_COMPILER;FABLE_COMPILER_3</DefineConstants>
     <TargetFrameworks>netstandard2.0;netstandard2.1</TargetFrameworks>