Update pnpm to v9.15.6

Author: renovate[bot]

package.json

@@ -23,7 +23,7 @@
     "prettier-plugin-ember-template-tag": "^2.0.0",
     "release-plan": "^0.11.0"
   },
-  "packageManager": "pnpm@9.11.0",
+  "packageManager": "pnpm@9.15.6",
   "engines": {
     "node": ">= 22.11.0"
   },