refactor!: use the KeyboardObserver package

[johnnewman-square]

This removes the internal KeyboardObserver and replaces it with https://github.com/square/swift-keyboard-observer. This also removes SetupKeyboardObserverOnAppStartup.m since we no longer support CocoaPods.

There will be some downstream changes required by consumers that I'm working on separately.

Checklist

Please do the following before merging:

• Ensure any public-facing changes are reflected in the changelog. Include them in the Main section.
• Update to an official release of KeyboardObserver.