Issue #3421849 by Wim Leers: Add validation constraints to announcements_feed.settings

core/modules/announcements_feed/config/schema/announcements_feed.schema.yml

 announcements_feed.settings:
   type: config_object
   label: 'Announcements Settings'
+  constraints:
+    FullyValidatable: ~
   mapping:
     max_age:
       type: integer
       label: 'Cache announcements for max-age seconds.'
+      constraints:
+        Range:
+          min: 0
     cron_interval:
       type: integer
       label: 'Cron interval for fetching announcements in seconds.'
+      constraints:
+        Range:
+          min: 0
     limit:
       type: integer
       label: 'Number of announcements that will be displayed.'
+      constraints:
+        Range:
+          min: 0