bump up iOS version to allow to compile with Xcode

Author: sebsto

Package.swift

@@ -6,7 +6,7 @@ import PackageDescription
 let package = Package(
     name: "swift-openapi-lambda",
     platforms: [
-        .macOS(.v12), .iOS(.v13), .tvOS(.v13), .watchOS(.v6),
+        .macOS(.v12), .iOS(.v15), .tvOS(.v15), .watchOS(.v8),
     ],
     products: [.library(name: "OpenAPILambda", targets: ["OpenAPILambda"])],
     dependencies: [