Swift: Update getting started example to build on Mac (awsdocs#6061)

Update example for getting started to build on Mac

Fixed some minor issues preventing Mac builds of the ListBuckets-Simple S3
example. These changes will need to be replicated across all examples, but
this is being pushed first because it is a dependency of a documentation
update. This example is embedded in fragments inline in the Swift SDK
developer guide's "Get started" page.

*Changes*

* Added the `platforms` block to the `Package.swift` file. This is needed to
  build for Apple platforms, and was removed from all examples by mistake
  during a cleanup pass many months ago.
* Renamed the `main.swift` file to `entry.swift`. This works around a known
  bug in the Swift build chain when the main program uses asynchronous code.

swift/example_code/s3/ListBuckets-Simple/Package.swift

@@ -11,12 +11,19 @@ import PackageDescription
 
 let package = Package(
     name: "ListBuckets-Simple",
+    // snippet-start:[s3.swift.intro.package-platforms]
+    // Let Xcode know the minimum Apple platforms supported.
+    platforms: [
+        .macOS(.v11),
+        .iOS(.v13)
+    ],
+    // snippet-end:[s3.swift.intro.package-platforms]
     // snippet-start:[s3.swift.intro.package-dependencies]
     dependencies: [
         // Dependencies declare other packages that this package depends on.
         .package(
             url: "https://github.com/awslabs/aws-sdk-swift",
-            from: "0.17.0"
+            from: "0.32.0"
         )
     ],
     // snippet-end:[s3.swift.intro.package-dependencies]