Re-add accidentally removed import of System.Diagnostics.CodeAnalysis

Author: MatthiasWerning

projects/RabbitMQ.Client/client/api/ICredentialsRefresher.cs

@@ -30,8 +30,8 @@
 //---------------------------------------------------------------------------
 
 using System;
-using System.Collections.Concurrent;
 using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
 using System.Diagnostics.Tracing;
 using System.Threading.Tasks;
 using System.Timers;