build(deps): bump zstandard from 0.20.0 to 0.21.0 in /backend (#1853)

Bumps [zstandard](https://github.com/indygreg/python-zstandard) from 0.20.0 to 0.21.0.
- [Release notes](https://github.com/indygreg/python-zstandard/releases)
- [Changelog](https://github.com/indygreg/python-zstandard/blob/main/docs/news.rst)
- [Commits](indygreg/python-zstandard@0.20.0...0.21.0)

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

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

Author: dependabot[bot]

backend/requirements.txt

@@ -10,4 +10,4 @@ python-dateutil==2.8.2
 pytoml==0.1.21
 redis==4.5.1
 swagger-ui-bundle==0.0.9
-zstandard==0.20.0
+zstandard==0.21.0