Revert C# language version back to 13.0 in Sixel.csproj

Co-authored-by: trackd <17672644+trackd@users.noreply.github.com>

Author: Copilot

src/Sixel/Sixel.csproj

@@ -5,7 +5,7 @@
     <ImplicitUsings>enable</ImplicitUsings>
     <Nullable>enable</Nullable>
     <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
-    <LangVersion>12.0</LangVersion>
+    <LangVersion>13.0</LangVersion>
     <Version>0.5.0</Version>
     <AnalysisMode>Recommended</AnalysisMode>
     <EnableNETAnalyzers>true</EnableNETAnalyzers>