Bump symfony/serializer from 6.4.13 to 6.4.15 in /app

Bumps [symfony/serializer](https://github.com/symfony/serializer) from 6.4.13 to 6.4.15.
- [Release notes](https://github.com/symfony/serializer/releases)
- [Changelog](https://github.com/symfony/serializer/blob/7.2/CHANGELOG.md)
- [Commits](symfony/serializer@v6.4.13...v6.4.15)

---
updated-dependencies:
- dependency-name: symfony/serializer
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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