[FirebaseAuth] Preconcurrency warning building with Xcode 16

[paulb777]

Description

See also https://github.com/swiftlang/swift/issues/74904

Reproducing the issue

No response

Firebase SDK Version

11.1.0

Xcode Version

16.0-beta5

Installation Method

CocoaPods

Firebase Product(s)

Authentication

Targeted Platforms

iOS

Relevant Log Output

Adding `@MainActor` as described at https://github.com/swiftlang/swift/issues/74904#issuecomment-2289909804 will address the warning, but will likely break existing public synchronous APIs.

If using Swift Package Manager, the project's Package.resolved

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If using CocoaPods, the project's Podfile.lock

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