chore(librarian): block generation for google-cloud-firestore due to failing system tests (#16507)

This is needed to unblock
#16500 which is
the weekly librarian generation PR to update generated code in this
repository. We can restore generation for google-cloud-firestore once
#16506 is fixed.

Author: parthea

.librarian/config.yaml

@@ -24,3 +24,7 @@ libraries:
 # Allow generation for google-cloud-bigtable once this bug is fixed.
   - id: "google-cloud-bigtable"
     generate_blocked: true
+# TODO(https://github.com/googleapis/google-cloud-python/issues/16506):
+# Allow generation for google-cloud-firestore once this bug is fixed.
+  - id: "google-cloud-firestore"
+    generate_blocked: true