[AIRFLOW-5897] Allow setting -1 as pool slots value in webserver (#6550)

This is a follow-up to 5d92162.

The original fix only applied to the www UI not the www_rbac one.

(cherry picked from commit 03a46dd)

airflow/www_rbac/views.py

@@ -2382,7 +2382,7 @@ def fqueued_slots(attr):
 
     validators_columns = {
         'pool': [validators.DataRequired()],
-        'slots': [validators.NumberRange(min=0)]
+        'slots': [validators.NumberRange(min=-1)]
     }