Update .background QoS to .utility. (#3207)

Source/EventMonitor.swift

@@ -310,7 +310,7 @@ extension EventMonitor {
 
 /// An `EventMonitor` which can contain multiple `EventMonitor`s and calls their methods on their queues.
 public final class CompositeEventMonitor: EventMonitor {
-    public let queue = DispatchQueue(label: "org.alamofire.compositeEventMonitor", qos: .background)
+    public let queue = DispatchQueue(label: "org.alamofire.compositeEventMonitor", qos: .utility)
 
     let monitors: [EventMonitor]
 

Tests/NSLoggingEventMonitor.swift

@@ -26,7 +26,7 @@ import Alamofire
 import Foundation
 
 public final class NSLoggingEventMonitor: EventMonitor {
-    public let queue = DispatchQueue(label: "org.alamofire.nsLoggingEventMonitorQueue", qos: .background)
+    public let queue = DispatchQueue(label: "org.alamofire.nsLoggingEventMonitorQueue", qos: .utility)
 
     public init() {}