Fix Package.swift Deprecation Warning (#3911)

### Issue Link :link:
Fixes #3908

### Goals :soccer:
This PR fixes the deprecation in the Package.swift file.

Package.swift

@@ -49,4 +49,4 @@ let package = Package(name: "Alamofire",
                                             path: "Tests",
                                             exclude: ["Info.plist", "Test Plans"],
                                             resources: [.process("Resources")])],
-                      swiftLanguageVersions: [.v5])
+                      swiftLanguageModes: [.v5])