chore: migrate firestore to java-firestore (#6808)
chore: migrate firestore to java-firestore (#6808)

* build: remove firestore kokoro configs
* chore: remove google-cloud-firestore after migration to java-firestore
* chore: update firestore client url in README
* chore(firestore): add READMEs after migrating to java-firestore

googleapis/java-cloud-bom,googleapis/java-cloud-bom,googleapis/java-cloud-bom,googleapis/java-cloud-bom