needs LangVersion 7.1 for 'default literal'; otherwise error CS8107

Index: modules/mono/glue/GodotSharp/GodotSharp/GodotSharp.csproj
--- modules/mono/glue/GodotSharp/GodotSharp/GodotSharp.csproj.orig
+++ modules/mono/glue/GodotSharp/GodotSharp/GodotSharp.csproj
@@ -7,6 +7,7 @@
     <TargetFramework>netstandard2.0</TargetFramework>
     <DocumentationFile>$(OutputPath)/$(AssemblyName).xml</DocumentationFile>
     <EnableDefaultItems>false</EnableDefaultItems>
+    <LangVersion>7.1</LangVersion>
   </PropertyGroup>
   <PropertyGroup>
     <DefineConstants>$(DefineConstants);GODOT</DefineConstants>
