Migrate to C#12

Util/Audio/Clip/AudioClipResolver.cs

@@ -54,7 +54,7 @@ internal static class AudioClipResolver
         }
 
         if (!clip) return store.GetOrAdd(key, () => null);
-        return store.GetOrAdd(key, () => UnityClipLoader.Load(clip!, store));
+        return store.GetOrAdd(key, () => UnityClipLoader.Load(clip, store));
     }
 
     /// <summary>Opens <paramref name="path"/> with FFmpeg; mono MP3 files are wrapped in <see cref="MP3MonoLayoutReader"/> (game layout).</summary>