Update mergify configuration. (backport #7385) (#7387)

Per https://blog.mergify.com/strict-mode-deprecation/, the strict mode
has been deprecated and will be turned off on 10-Jan-2022. This updates
the config to use the new, approved thing instead of the old thing.

(cherry picked from commit 2d4844f97f)

.github/mergify.yml

@@ -1,18 +1,16 @@
+queue_rules:
+  - name: default
+    conditions:
+      - base=v0.35.x
+      - label=S:automerge
+
 pull_request_rules:
-  - name: Automerge to master
+  - name: Automerge to v0.35.x
     conditions:
-      - base=master
+      - base=v0.35.x
       - label=S:automerge
     actions:
-      merge:
+      queue:
         method: squash
-        strict: smart+fasttrack
+        name: default
         commit_message: title+body
-  - name: backport patches to v0.34.x branch
-    conditions:
-      - base=master
-      - label=S:backport-to-v0.34.x
-    actions:
-      backport:
-        branches:
-          - v0.34.x