Don't dispose managed CancellationTokenSource (#6139)

Jellyfin.Api/Controllers/VideosController.cs

@@ -373,6 +373,7 @@ namespace Jellyfin.Api.Controllers
             [FromQuery] Dictionary<string, string> streamOptions)
         {
             var isHeadRequest = Request.Method == System.Net.WebRequestMethods.Http.Head;
+            // CTS lifecycle is managed internally.
             var cancellationTokenSource = new CancellationTokenSource();
             var streamingRequest = new VideoRequestDto
             {