Bump minimum iOS version to 12

[mdmathias]

Context

Apple requires that a minimum of Xcode 15 is used for App Store submissions. Xcode 15's minimum supported iOS version is 12. Currently, GSI supports iOS 11.

To Do

Given the requirement to submit apps built with Xcode 15, we should increment GSI's baseline iOS support to 12.

NB

This will be a breaking change since clients will need to update their Package.swift and Podfile files if they previously supported iOS 11. Therefore, this needs to be included in a version 8.0.0 release of this library.