[FirebaseAuth] Preconcurrency warning building with Xcode 16

[paulb777]

Description

See also swiftlang/swift#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

Expand Package.resolved snippet

Replace this line with the contents of your Package.resolved.

If using CocoaPods, the project's Podfile.lock

Expand Podfile.lock snippet

Replace this line with the contents of your Podfile.lock!