ci: pin automergeStrategy to squash

The default, auto, follows the repository's platform merge strategy. Every
ruleset in the fleet now requires linear history while still allowing merge
commits, so an auto choice of merge-commit would be rejected and the PR would
sit unmerged with all checks green. squash is permitted by every ruleset and
satisfies linear history.

Author: bakerkj

renovate.json

@@ -114,5 +114,6 @@
       "automerge": false
     }
   ],
-  "prConcurrentLimit": 5
+  "prConcurrentLimit": 5,
+  "automergeStrategy": "squash"
 }