* Enable async/await APIs for iOS 13, etc.
This bumps the required compiler version to 5.5.2 which was included in
Xcode 13.2, where backporting was first introduced.
This breaks less developers and enables developers who aren't using
these APIs to still compile using Xcode 13.0 and 13.1.
As far as I can tell, the only breakage this includes now is developers
who are using Xcode 13.0 or 13.1, targeting iOS 15 and using one of
these 7 APIs. It will result in a compiler error and they'll have to
either use the completion handler version or upgrade to Xcode 13.2+ to
fix the compilation error.
* Missed checks, CHANGELOG
* Fixed avialability, added watchOS for RTDB
* Moved to watchOS 7 for all async/await APIs to match platform support
