Lock package manager version (elastic#175495)

## Summary

I believe the intention was to restrict the `yarn` version to `1.22.19`,
but the current `package.json` setup does not enforce it. This config
change should do it.


### Checklist

None.

### Risk Matrix

None.

### For maintainers

- [ ] This was checked for breaking API changes and was [labeled
appropriately](https://www.elastic.co/guide/en/kibana/master/contributing.html#kibana-release-notes-process)

---------

Co-authored-by: Jon <jon@budzenski.me>
Co-authored-by: Jon <jon@elastic.co>

package.json

@@ -75,7 +75,7 @@
   },
   "engines": {
     "node": "20.10.0",
-    "yarn": "^1.22.19"
+    "yarn": "^1.22.21"
   },
   "resolutions": {
     "**/@hello-pangea/dnd": "16.2.0",
@@ -1684,5 +1684,6 @@
     "xmlbuilder": "13.0.2",
     "yargs": "^15.4.1",
     "yarn-deduplicate": "^6.0.2"
-  }
+  },
+  "packageManager": "yarn@1.22.21"
 }