Update to latest version of C# (14)

Author: atifaziz

Directory.Build.props

@@ -1,7 +1,7 @@
 <Project>
 
   <PropertyGroup>
-    <LangVersion>10</LangVersion>
+    <LangVersion>14</LangVersion>
     <Nullable>enable</Nullable>
     <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
     <!--

src/DocoptNet/StringList.cs

@@ -27,7 +27,7 @@ public static StringList BottomTop(params string[] items) =>
             items.Aggregate(Empty, (stack, item) => stack.Push(item));
 
         public static StringList TopBottom(params string[] items) =>
-            items.Reverse().Aggregate(Empty, (stack, item) => stack.Push(item));
+            items.AsEnumerable().Reverse().Aggregate(Empty, (stack, item) => stack.Push(item));
 
         StringList(string value, StringList next) : this(value, next, next.Count + 1) { }