disable bot notification spam (#68460)

* disable bot notification spam

* add new key to botmeta sanity

.github/BOTMETA.yml

@@ -1,4 +1,5 @@
 automerge: false
+notifications: false
 files:
   .github/BOTMETA.yml:
     labels: botmeta

test/sanity/code-smell/botmeta.py

@@ -53,6 +53,7 @@ def main():
 
     schema = Schema({
         Required('automerge'): bool,
+        Required('notifications'): bool,
         Required('files'): Any(None, *list_dict_file_schema),
         Required('macros'): dict,  # Any(*list_macros_schema),
     })