Gate SwitchOnReadOnlySpanChar on C# 11.0

The setting carried the C# 11.0 display category but was missing from
both SetLanguageVersion and GetMinimumRequiredVersion, so decompiling
for an older target language version could still produce switches over
ReadOnlySpan<char> that the requested compiler cannot compile. Gating
it like the other C# 11.0 settings closes that gap; the category string
is now derived from the version like everywhere else.

Assisted-by: Claude:claude-fable-5:Claude Code

ICSharpCode.Decompiler/DecompilerSettings.cs

@@ -483,9 +483,8 @@ namespace ICSharpCode.Decompiler
 		/// <summary>
 		/// Gets/Sets whether to use C# 11.0 switch on (ReadOnly)Span&lt;char&gt;
 		/// </summary>
-		[Category("C# 11.0 / VS 2022.4")]
 		[Description("DecompilerSettings.SwitchOnReadOnlySpanChar")]
-		[DecompilerSetting]
+		[DecompilerSetting(CSharp.LanguageVersion.CSharp11_0)]
 		public partial bool SwitchOnReadOnlySpanChar { get; set; }
 
 		/// <summary>