firestore: release v1.2.0
This commit will be tagged firestore/v1.2.0.
# Changes
+## v1.2.0
+
+- Deprecate v1beta1 client.
+- Fix serverTimestamp docs.
+- Add missing operators to query docs.
+- Make document IDs 20 alpha-numeric characters. Previously, there could be more
+ than 20 non-alphanumeric characters, which broke some users. See
+ https://github.com/googleapis/google-cloud-go/issues/1715.
+- Various updates to autogenerated clients.
+
## v1.1.1
- Fix bug in CollectionGroup query validation.
