build(deps-dev): bump fakeredis from 2.20.0 to 2.21.1 in /backend (#2262)

Bumps [fakeredis](https://github.com/cunla/fakeredis-py) from 2.20.0 to 2.21.1.
- [Release notes](https://github.com/cunla/fakeredis-py/releases)
- [Commits](cunla/fakeredis-py@v2.20.0...v2.21.1)

---
updated-dependencies:
- dependency-name: fakeredis
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

backend/requirements-dev.txt

@@ -1,4 +1,4 @@
-fakeredis==2.20.0
+fakeredis==2.21.1
 pre-commit==3.5.0
 pytest==8.0.2
 pytest-responses==0.5.1