⬇️(dependencies) downgrade next from 16.0.10 to 15.5.9

Passing the Next 16 will need more work to be compatible
with our application. We will do this upgrade later
in a dedicated PR.
We add it in the renovate.json to avoid having Renovate
trying to upgrade it again.

src/frontend/packages/eslint-plugin-docs/package.json

@@ -17,12 +17,12 @@
     "eslint": ">=9.0.0"
   },
   "dependencies": {
-    "@next/eslint-plugin-next": "16.0.10",
+    "@next/eslint-plugin-next": "15.5.9",
     "@tanstack/eslint-plugin-query": "5.91.2",
     "@typescript-eslint/eslint-plugin": "8.49.0",
     "@typescript-eslint/parser": "8.49.0",
     "@vitest/eslint-plugin": "1.5.2",
-    "eslint-config-next": "16.0.10",
+    "eslint-config-next": "15.5.9",
     "eslint-config-prettier": "10.1.8",
     "eslint-plugin-import": "2.32.0",
     "eslint-plugin-jest": "29.5.0",