Update firestore to use FieldFilter.

Fix: #431
UserWarning in build log: Detected filter using positional arguments.
Prefer using the 'filter' keyword argument instead
The filter argument takes BaseFilter class which is an abstract class
and it is implemented in FieldFilter. This class can be imported from
google.cloud.firestore_v1.base_query
Reference: googleapis/python-firestore#705
	   GoogleCloudPlatform/python-docs-samples#10407

kcidb/monitor/spool/__init__.py

@@ -12,6 +12,7 @@
 import email
 import email.policy
 from google.cloud import firestore
+from google.cloud.firestore_v1.base_query import FieldFilter
 from kcidb.misc import ArgumentParser, iso_timestamp, log_and_print_excepthook
 from kcidb.monitor.misc import is_valid_firestore_id
 from kcidb.monitor.output import Notification
@@ -238,8 +239,9 @@ def wipe(self, until=None):
             isinstance(until, datetime.datetime) and until.tzinfo
         if until is None:
             until = datetime.datetime.now(datetime.timezone.utc)
+        field_filter = FieldFilter("created_at", "<=", until)
         for snapshot in \
-            self._get_coll().where("created_at", "<=", until). \
+            self._get_coll().where(filter=field_filter). \
                 select([]).stream():
             snapshot.reference.delete()
 
@@ -259,8 +261,9 @@ def unpicked(self, timestamp=None):
             isinstance(timestamp, datetime.datetime) and timestamp.tzinfo
         if timestamp is None:
             timestamp = datetime.datetime.now(datetime.timezone.utc)
+        field_filter = FieldFilter("picked_until", "<", timestamp)
         for snapshot in \
-            self._get_coll().where("picked_until", "<", timestamp). \
+            self._get_coll().where(filter=field_filter). \
                 select([]).stream():
             id = snapshot.id
             assert Client.is_valid_id(id)