Reduce minimum platform target to match OpenAPI Generator

Author: 0xTim

Package.swift

@@ -5,10 +5,10 @@ import PackageDescription
 let package = Package(
     name: "swift-openapi-vapor",
     platforms: [
-        .macOS(.v13),
-        .iOS(.v16),
-        .tvOS(.v16),
-        .watchOS(.v9)
+        .macOS(.v10_15),
+        .iOS(.v13),
+        .tvOS(.v13),
+        .watchOS(.v6)
     ],
     products: [
         .library(name: "OpenAPIVapor", targets: ["OpenAPIVapor"])