Update Xcode 12 requirement for SwiftPM docs. (#6491)
Update Xcode 12 requirement for SwiftPM docs. (#6491)

Since it's out of beta, let's just mention Xcode 12.
firebase/firebase-ios-sdk,firebase/firebase-ios-sdk,firebase/firebase-ios-sdk,firebase/firebase-ios-sdk,firebase/firebase-ios-sdk,firebase/firebase-ios-sdk,firebase/firebase-ios-sdk