Upgrade target to iOS 12 to allow build on Xcode 13

Package.swift

@@ -3,7 +3,7 @@ import PackageDescription
 
 let package = Package(
     name: "Kingfisher",
-    platforms: [.iOS(.v10), .macOS(.v10_12), .tvOS(.v10), .watchOS(.v3)],
+    platforms: [.iOS(.v12), .macOS(.v10_14), .tvOS(.v12), .watchOS(.v5)],
     products: [
         .library(name: "Kingfisher", targets: ["Kingfisher"])
     ],