Replace `WebClient` with `HttpClient` implementation

[alexrster]

core/ContactPoint.Plugins.PauseNotifications/PauseNotificationsPlugin.cs

Line 104 in 4999870

using (var client = new WebClient { CachePolicy = new RequestCachePolicy(RequestCacheLevel.NoCacheNoStore), Proxy = _proxy })

WebClient has been deprecated - replace it with HttpClient for better stability and performance