Update mergify configuration. (backport #7385) (#7386)

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

@@ -0,0 +1,16 @@
+queue_rules:
+  - name: default
+    conditions:
+      - base=v0.34.x
+      - label=S:automerge
+
+pull_request_rules:
+  - name: Automerge to v0.34.x
+    conditions:
+      - base=v0.34.x
+      - label=S:automerge
+    actions:
+      queue:
+        method: squash
+        name: default
+        commit_message: title+body