Renaming FirebaseAdapter to FirestoreAdapter

[chanceaclark]

Proposal

Based on #3873 it seems like the name of the adapter adds some confusion on the intention of what the adapter is trying to accomplish. Firebase is just a collection of different tools, Firestore, Auth, Hosting, etc..., and next-auth adapters have been primarily types of databases to store auth data. With that in mind, we should think of the FirebaseAdapter as just a connection to Firestore, as it was initially built as. Some my proposal is to rename the adapter from FirebaseAdapter to FirestoreAdapter.

What about Firebase Auth?

Firebase Auth is another beast that needs to get tackled separately. From my understanding, that tool is similar to next-auth, but provides its own database. For example, Firebase Auth has its own set of auth provider types, like next-auth has their own.