Add a Swift version check across all platforms. (#3549)

Source/Alamofire.swift

@@ -28,6 +28,10 @@ import Foundation
 @_exported import FoundationNetworking
 #endif
 
+#if swift(<5.3)
+#error("Alamofire doesn't support Swift versions below 5.3")
+#endif
+
 /// Reference to `Session.default` for quick bootstrapping and examples.
 public let AF = Session.default