Migrate to C#12

Util/Audio/SourceState.cs

@@ -76,7 +76,7 @@ internal sealed class SourceState
     public void BindClip(AudioClip? clip)
     {
         if (!clip) return;
-        _length = clip!.length;
+        _length = clip.length;
         _frequency = Math.Max(1, clip.frequency);
     }