feat: Bump Firebase iOS SDK to `9.6.0`

[russellwheatley]

Description

see title.

Related Issues

closes https://github.com/firebase/flutterfire/issues/8811

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[mikehardy]

The macOS failure looks unrelated:

Specs satisfying the `FirebaseFirestore (from `[https://github.com/invertase/firestore-ios-sdk-frameworks.git`](https://github.com/invertase/firestore-ios-sdk-frameworks.git%60), tag `9.6.0`)` dependency were found, but they required a higher minimum deployment target.

Here is the thing with that: the cocoapods integration of Firestore is using the pre-compiled framework which is based off the zip distribution, which has a minimum target of iOS 11 and macOS 10.13 -- see here https://github.com/invertase/firestore-ios-sdk-frameworks/issues/56#issuecomment-1228644812

Those are slightly higher than the cocoapods (not zip-based!) requirements for firestore.

So you either need to de-integrate the pre-compiled version and use the "normal" cocoapods integration for firestore, or you need to update the minimum target for iOS/macOS to get that CI check to pass

Note that as Paul B says, next version of firebase-ios-sdk will use the new minimums already in place for zip distribution, even on the cocoapods integration, so my suggested course for CI would be to go ahead and bump the minimum target since it will be necessary soon anyway, but that's a policy decision, and there may be good reasons not to.