Update mergify configuration. (#198)

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.

Author: M. J. Fromberger

.mergify.yml

@@ -1,9 +1,15 @@
+queue_rules:
+  - name: default
+    conditions:
+      - base=master
+      - label=S:automerge
+
 pull_request_rules:
   - name: automerge to master with label S:automerge and branch protection passing
     conditions:
       - base=master
       - label=S:automerge
     actions:
-      merge:
+      queue:
         method: squash
-        strict: true
+        name: default