chore: Specify C# version

Reikyo · web-flow · commit 11087454b88b · 2025-04-16T16:15:15.000+01:00
diff --git a/OpenActive.Server.NET/OpenActive.Server.NET.csproj b/OpenActive.Server.NET/OpenActive.Server.NET.csproj
@@ -2,6 +2,7 @@
 
   <PropertyGroup>
     <TargetFrameworks>netstandard2.0</TargetFrameworks>
+    <LangVersion>8.0</LangVersion>
     <Authors>OpenActive Community</Authors>
     <Company>OpenActive</Company>
     <Copyright>OpenActive</Copyright>
