Remove Cocoapods Support

Cocoapods will move to read-only on December 2, 2026 ([blogpost](https://blog.cocoapods.org/CocoaPods-Specs-Repo/)).

With that in mind, we should remove Cocoapods support and encourage clients to migrate before said deadline. The following will be required:

- Write, test, and publish migration instructions.
- Notify library users of the impending deprecation.
- Stage changes that remove support.

Additional:
Firebase has published instructions & their own pre-deadline [here](https://firebase.google.com/docs/ios/cocoapods-deprecation), and we'll likely be able to utilize the same.

Start with the CocoaPods deprecation announcement and Firebase's linked migration instructions. Define the migration guidance and user notification needed before the December 2, 2026 read-only deadline, then identify the staged changes required to remove CocoaPods support. Done means instructions are written, tested, and published, users are notified, and removal work is staged.

索引モデルが issue の本文から書いたものです。