mergify: drop ready for draft (#15766)

Author: Borda

.github/mergify.yml

@@ -50,6 +50,7 @@ pull_request_rules:
   - name: Not ready yet
     conditions:
       - or:
+        - draft # filter-out GH draft PRs
         - label="has conflicts"
         - "#approved-reviews-by=0" # number of review approvals
         - "#changes-requested-reviews-by>=1" # no requested changes