build(deps): bump zstandard from 0.15.1 to 0.15.2 in /backend

Bumps [zstandard](https://github.com/indygreg/python-zstandard) from 0.15.1 to 0.15.2.
- [Release notes](https://github.com/indygreg/python-zstandard/releases)
- [Changelog](https://github.com/indygreg/python-zstandard/blob/0.15.2/docs/news.rst)
- [Commits](indygreg/python-zstandard@0.15.1...0.15.2)

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

Author: dependabot[bot]

backend/requirements.txt

@@ -11,4 +11,4 @@ python-dateutil==2.8.1
 pytoml==0.1.21
 redis==3.5.3
 swagger-ui-bundle==0.0.8
-zstandard==0.15.1
+zstandard==0.15.2