🤖 renovate: Only update branch in case of conflicts

Renovate defaults to `behind-base-branch` strategy if automerge is
enabled (which we do now for non-major update). We don't need
unnecessary rebasing when there are no conflicts.

.github/renovate.json

@@ -13,6 +13,9 @@
     {
       "matchUpdateTypes": ["minor", "patch", "pin", "digest"],
       "automerge": true
+    },
+    {
+      "rebaseWhen": "conflicted"
     }
   ]
 }