Fix compile failure on iOS for Mac Catalyst support

[imxieyi]

This PR fixes compile failure on iOS, which happens if you are adding CLI support for a Mac Catalyst app. Specifically, it fixes:

'sortedKeys' is only available in iOS 11.0 or newer

Checklist

• [N/A] I've added at least one test that validates that my change is working, if appropriate
• I've followed the code style of the rest of the project
• I've read the Contribution Guidelines
• [N/A] I've updated the documentation if necessary

[rauhul]

@swift-ci please test

[rauhul]

It may be worth adding annotations for all platforms just in-case. https://developer.apple.com/documentation/foundation/jsonserialization/writingoptions/2888322-sortedkeys

[rauhul]

@swift-ci please test

[natecook1000]

Thanks, @imxieyi! 👏🏻