composer: bump phpbu/phpbu from 6.0.30 to 6.0.31 in /app

Bumps [phpbu/phpbu](https://github.com/sebastianfeldmann/phpbu) from 6.0.30 to 6.0.31.
- [Release notes](https://github.com/sebastianfeldmann/phpbu/releases)
- [Commits](sebastianfeldmann/phpbu@6.0.30...6.0.31)

---
updated-dependencies:
- dependency-name: phpbu/phpbu
  dependency-version: 6.0.31
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]