⬇️(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/apps/impress/package.json

@@ -57,7 +57,7 @@
     "idb": "8.0.3",
     "lodash": "4.17.21",
     "luxon": "3.7.2",
-    "next": "16.0.10",
+    "next": "15.5.9",
     "posthog-js": "1.306.1",
     "react": "*",
     "react-aria-components": "1.13.0",