mergify: update YAML config

Problem: Mergify is complaining that the setting to require branches to
be up to date before merging is not compatible with draft PR checks.

Update the Mergify configuration to enable in-place checks by setting
merge_queue.max_parallel_checks to 1 and setting every queue_rule's
batch_size to 1.

Author: cmoussa1

.mergify.yml

@@ -1,29 +1,25 @@
 queue_rules:
   - name: default
-    queue_conditions:
-      - base=master
-      - status-success="validate commits"
-      - label="merge-when-passing"
-      - label!="work-in-progress"
-      - "approved-reviews-by=@flux-framework/docs"
-      - "#approved-reviews-by>0"
-      - "#changes-requested-reviews-by=0"
-      - -title~=^\[*[Ww][Ii][Pp]
-    merge_conditions:
+    batch_size: 1
+    update_method: rebase
+    merge_method: merge
+
+# Avoid temporary branches created by mergify for parallel checks
+# These do not work with the pr-validator since the temporary PR
+# branch is updated with a merge commit.
+merge_queue:
+  max_parallel_checks: 1
+
+pull_request_rules:
+  - name: rebase and merge when passing all checks
+    conditions:
       - base=master
-      - status-success="docs/readthedocs.org:flux-framework"
-      - status-success="validate commits"
       - label="merge-when-passing"
       - label!="work-in-progress"
-      - "approved-reviews-by=@flux-framework/docs"
+      - -title~="^\[*(WIP|wip)"
+      - "approved-reviews-by=@flux-framework/core"
       - "#approved-reviews-by>0"
       - "#changes-requested-reviews-by=0"
-      - -title~=^\[*[Ww][Ii][Pp]
-    merge_method: merge
-    update_method: rebase
-
-pull_request_rules:
-  - name: refactored queue action rule
-    conditions: []
     actions:
       queue:
+        name: default