Use SwiftPM default minimum deployment targets
Use SwiftPM default minimum deployment targets

Xcode 12 raises the minimum known iOS deployment target from iOS 8.0 to
iOS 9.0, resulting in a warning when using RxSwift via SwiftPM. Instead
of explicitly specifying minimum deployment targets, we can use
SwiftPM's default values which will stay up-to-date whenever the
toolchain in use changes the valid deployment target range.

ReactiveX/RxSwift,kzaher/RxSwift,kzaher/RxSwift,kzaher/RxSwift,ReactiveX/RxSwift,ReactiveX/RxSwift,kzaher/RxSwift