[FR]: Update AppCheck dependency library's minimum supported versions

[paulb777]

Description

Update https://github.com/google/app-check/blob/main/Package.swift#L22 to iOS 15 or at least iOS 13 to enable async/await modernization.

Will require coordination with GoogleSignIn https://github.com/google/GoogleSignIn-iOS/blob/main/Package.swift#L27

API Proposal

No response

Firebase Product(s)

App Check

[paulb777]

In progress at google/app-check#111 and google/GoogleSignIn-iOS#626