Update SPM deployment target to iOS 13

Author: heoblitz

Package.swift

@@ -1,12 +1,12 @@
-// swift-tools-version:5.3
+// swift-tools-version:5.5
 // The swift-tools-version declares the minimum version of Swift required to build this package.
 
 import PackageDescription
 
 let package = Package(
   name: "FlexLayout",
   platforms: [
-    .iOS(.v12),
+    .iOS(.v13),
   ],
   products: [
     .library(name: "FlexLayout", targets: ["FlexLayout"]),