Update FirebaseMessaging Dependency in v2 Branch for Compatibility with New Firebase Versions

[AristideVB]

Description:

This pull request updates the FirebaseMessaging dependency in the v2 branch of the Customer.io iOS SDK to ensure compatibility with Firebase versions up to but not including 12.0.0.

Reason for the Update:

• The Customer.io Flutter plugin currently depends on the v2 iOS SDK and has not yet been updated to support the v3 iOS SDK.
• To avoid waiting for the Flutter plugin to be updated, we need to ensure that the v2 iOS SDK remains compatible with the latest Firebase versions, those latest Firebase versions include fixes for building with Xcode 16.

Impact:

• This update ensures that projects using the Customer.io Flutter plugin (and thus the v2 iOS SDK) can build successfully with the latest versions of Firebase and thus with Xcode 16.
• The update preserves compatibility with existing functionality in the v2.x SDK, while allowing newer Firebase versions to be used.

Updated the range to >8.7.0 <12.0.0 and tested on UIKit and CocoaPod sample apps. There are no major breaking changes as per the official Firebase document that affects this SDK.

Closes #823

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