fix 6.0 language mode

Author: sebsto

[email protected]

@@ -131,7 +131,8 @@ let package = Package(
             dependencies: [
                 .product(name: "NIOHTTP1", package: "swift-nio"),
                 .product(name: "NIOCore", package: "swift-nio"),
-            ]
+            ],
+            swiftSettings: defaultSwiftSettings
         ),
         .testTarget(
             name: "AWSLambdaRuntimeTests",