fix(apple): build on Firebase Apple SDK 13 and require macOS 11 for Swift packages

[SelaseKay]

Description

Stacked on #18771. The base branch is feat/bump-ios-sdk-13.0.0, not main, because these changes only compile against the Firebase Apple SDK 13 Swift Package Manager pin. Merge this into the bump branch first; that turns #18771 green, and then it merges into main.

• Firestore pipelines. Firestore 13 prefixes the pipeline bridge classes. The same parser is still built by CocoaPods against Firebase Apple SDK 12.19.0 and its unprefixed names, so the Swift package sets FLT_FIREBASE_SDK_13 and the parser picks the right names for each path.
• ML Model Downloader. The Firebase Apple SDK 13 Swift package no longer ships FirebaseMLModelDownloader. With Swift Package Manager, the plugin now depends on FirebaseCore only and returns an unimplemented error for every method call. CocoaPods stays on SDK 12.19.0 and keeps working, and the ML Model Downloader end-to-end workflow runs on CocoaPods.
• macOS 11. Firebase Apple SDK 13 requires macOS 11, so the macOS Package.swift files move from 10.15 to 11. CocoaPods podspecs keep 10.15. The iOS deployment target is unchanged at 15.0.
• Formatting for the parser and the model downloader guard.
• Auth iOS end-to-end on CocoaPods. Under Swift Package Manager, Firebase Apple SDK 13 requires App Check 12, but every GoogleSignIn-iOS release up to 10.0.0 requires App Check 11, so the auth example's google_sign_in cannot resolve. Bumping google_sign_in does not help: the latest google_sign_in_ios still lands on App Check 11. The auth iOS job now runs on CocoaPods (still SDK 12.19.0) until [App Check] Add SPM AppCheckCore 12.x support google/GoogleSignIn-iOS#626 ships and google_sign_in_ios accepts that release. Until then, no end-to-end job builds firebase_auth under Swift Package Manager on SDK 13.

Related Issues

• Part of 🚀 [firebase_core] Bump Firebase iOS SDK (12.19.0 → 13.0.0) #18769

Checklist

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• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• My PR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide).
• All existing and new tests are passing.
• I updated/added relevant documentation (doc comments with ///).
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Breaking Change

Does your PR require plugin users to manually update their apps to accommodate your change?

• Yes, this is a breaking change.
• No, this is not a breaking change.

Swift Package Manager builds require macOS 11, and firebase_ml_model_downloader is unavailable with Swift Package Manager. The breaking-change marker for the release lives on the title of #18771, because this pull request is squashed into that branch.

Test plan

• CI on this pull request runs the Apple jobs for the files it touches.
• After it merges, the full CI run on feat!: bump Firebase iOS SDK to 13.0.0 #18771 should be green, with auth iOS end-to-end running on CocoaPods.

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