chore: graduate Firestore Pipelines features to GA

This audit and cleanup removes the @beta tags from most Firestore Pipelines
SDK features, marking them as GA (General Availability).

Graduated features include:
- Pipeline entry point and source stages (collection, documents, etc.)
- Nearly all pipeline expressions (logical, string, numeric, date, etc.)
- Aggregations (except grouped aggregations which remain GA-bound)
- findNearest stage and its associated options
- explain and analyze options

The following remain as beta, as per requirements:
- DML stages (delete, update)

Author: wu-hui