Bump validator from 13.15.15 to 13.15.20 in /backend

Bumps [validator](https://github.com/validatorjs/validator.js) from 13.15.15 to 13.15.20.
- [Release notes](https://github.com/validatorjs/validator.js/releases)
- [Changelog](https://github.com/validatorjs/validator.js/blob/master/CHANGELOG.md)
- [Commits](validatorjs/validator.js@13.15.15...13.15.20)

---
updated-dependencies:
- dependency-name: validator
  dependency-version: 13.15.20
  dependency-type: indirect
...

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

Author: dependabot[bot]