chore(deps): bump next-i18next in /frontend/kesaseteli/youth

Bumps [next-i18next](https://github.com/i18next/next-i18next) from 13.3.0 to 15.4.0.
- [Release notes](https://github.com/i18next/next-i18next/releases)
- [Changelog](https://github.com/i18next/next-i18next/blob/master/CHANGELOG.md)
- [Commits](i18next/next-i18next@v13.3.0...v15.4.0)

---
updated-dependencies:
- dependency-name: next-i18next
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <[email protected]>

frontend/kesaseteli/youth/package.json

@@ -31,7 +31,7 @@
     "lodash": "^4.17.21",
     "next": "14.2.12",
     "next-compose-plugins": "^2.2.1",
-    "next-i18next": "^13.0.3",
+    "next-i18next": "^15.4.0",
     "next-router-mock": "0.9.12",
     "react": "^18.2.0",
     "react-dom": "^18.2.0",