Update Package.swift files (#7039)

This commit updates Package.swift files that were missing the platform
specification that indicates that the targets can be built for iOS and
macOS. This was preventing the examples from working on these platforms.

Author: shepazon

swift/example_code/swift-sdk/Waiters/Package.swift

@@ -9,6 +9,11 @@ import PackageDescription
 
 let package = Package(
     name: "Waiters",
+    // Let Xcode know the minimum Apple platforms supported.
+    platforms: [
+        .macOS(.v11),
+        .iOS(.v13)
+    ],
     dependencies: [
         // This project requires the AWS SDK for Swift.
         .package(

swift/example_code/swift-sdk/mocking/Package.swift

@@ -9,6 +9,11 @@ import PackageDescription
 
 let package = Package(
     name: "mocking",
+    // Let Xcode know the minimum Apple platforms supported.
+    platforms: [
+        .macOS(.v11),
+        .iOS(.v13)
+    ],
     dependencies: [
         // Dependencies declare other packages that this package depends on.
         .package(

swift/modules/SwiftUtilities/Package.swift

@@ -7,6 +7,11 @@ import PackageDescription
 
 let package = Package(
     name: "SwiftUtilities",
+    // Let Xcode know the minimum Apple platforms supported.
+    platforms: [
+        .macOS(.v11),
+        .iOS(.v13)
+    ],
     products: [
         // Products define the executables and libraries a package produces.
         // They also make their own executables and libraries visible to other