Bump express-validator from 7.0.1 to 7.1.0 in /backend

Bumps [express-validator](https://github.com/express-validator/express-validator) from 7.0.1 to 7.1.0.
- [Release notes](https://github.com/express-validator/express-validator/releases)
- [Commits](express-validator/express-validator@v7.0.1...v7.1.0)

---
updated-dependencies:
- dependency-name: express-validator
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

Author: dependabot[bot]