React to new analyzer warnings

Author: captainsafia

src/HealthChecks/HealthChecks/src/HealthCheckPublisherHostedService.cs

@@ -20,7 +20,7 @@ internal sealed class HealthCheckPublisherHostedService : IHostedService
         private readonly ILogger _logger;
         private readonly IHealthCheckPublisher[] _publishers;
 
-        private CancellationTokenSource _stopping;
+        private readonly CancellationTokenSource _stopping;
         private Timer? _timer;
         private CancellationTokenSource? _runTokenSource;