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 by reviewing the repository's existing CocoaPods support and the linked Firebase migration instructions. Document the migration path, notify library users about the deprecation, and stage the support-removal changes; done means all three requested workstreams are prepared and tested.

Written by the indexing model from the issue text.