Migrate to C#12

Util/Audio/Clip/AudioClipStore.cs

@@ -198,7 +198,7 @@ internal sealed class AudioClipStore : IDisposable
         // same key meanwhile, its entry wins and this source is discarded.
         lock (_sources)
         {
-            if (_disposed || _sources.TryGetValue(key, out var raced))
+            if (_disposed || _sources.TryGetValue(key, out _))
             {
                 source?.DisposeInternal();
                 return _disposed ? null : _sources[key];