Bump lodash from 4.17.20 to 4.17.21 in /backend

Bumps [lodash](https://github.com/lodash/lodash) from 4.17.20 to 4.17.21.
- [Release notes](https://github.com/lodash/lodash/releases)
- [Commits](lodash/lodash@4.17.20...4.17.21)

---
updated-dependencies:
- dependency-name: lodash
  dependency-type: indirect
...

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Author: dependabot[bot]