Bump knex from 0.21.2 to 2.4.0 in /backend

Bumps [knex](https://github.com/knex/knex) from 0.21.2 to 2.4.0.
- [Release notes](https://github.com/knex/knex/releases)
- [Changelog](https://github.com/knex/knex/blob/master/CHANGELOG.md)
- [Commits](knex/knex@0.21.2...2.4.0)

---
updated-dependencies:
- dependency-name: knex
  dependency-type: direct:production
...

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Author: dependabot[bot]