Update async/await code to be available for iOS 13, etc.

[paulb777]

Now that it is supported for iOS 13 instead of 15 starting with Xcode 13.2.1

[ryanwilson]

This will bump our min Xcode version to Xcode 13.2.1 right?

[paulb777]

Right.

Seems worth it for the next major release to get the broader async/await support and Xcode 13.2.1+ usage is already 63%

[ryanwilson]

I wonder if we could use #if swift(>=5.5.2)` to check for it and set it to 13, otherwise set it to 15. May be a bit messy but could buy us a bit more time and we don't have that many instances of it.

The one caveat is 13.2.0 specifically would break I think, but if we were thinking of removing support anyways it's not a huge deal.

[paulb777]

Maybe, but is the complexity/maintainability worth it? I think most people will update their Xcode versions quicker than Firebase major versions.

[ryanwilson]

Testing this out now as there are only 7 non-test instances of these declarations.

[ryanwilson]

This is fixed in #9540