Migrate to C#12

Util/Audio/OfflineAudioMixer.cs

@@ -164,7 +164,7 @@ internal sealed class OfflineAudioMixer : IDisposable
             return;
         }
 
-        _bgmKey = clip!.name;
+        _bgmKey = clip.name;
         _bgmSource = AudioClipResolver.GetOrAdd(_store, _bgmKey, clip);
         if (_bgmSource == null)
         {
@@ -268,7 +268,7 @@ internal sealed class OfflineAudioMixer : IDisposable
 
         var state = new SourceState(voice, Math.Max(startTime, now), pitch, loop, loop ? endTime : double.PositiveInfinity,
             clipSource?.Duration ?? 0.0, _sampleRate);
-        if (source) MapSource(source!, state);
+        if (source) MapSource(source, state);
         return state;
     }