Bump redis[hiredis] from 5.0.2 to 6.4.0 in /backend

Bumps [redis[hiredis]](https://github.com/redis/redis-py) from 5.0.2 to 6.4.0.
- [Release notes](https://github.com/redis/redis-py/releases)
- [Changelog](https://github.com/redis/redis-py/blob/master/CHANGES)
- [Commits](redis/redis-py@v5.0.2...v6.4.0)

---
updated-dependencies:
- dependency-name: redis[hiredis]
  dependency-version: 6.4.0
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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

Author: dependabot[bot]

backend/pyproject.toml

@@ -75,7 +75,7 @@ dependencies = [
     "django-markdownify==0.9.5",
     "django-imagekit==5.0.0",
     "pillow==10.4.0",
-    "redis[hiredis]==5.0.2",
+    "redis[hiredis]==6.4.0",
     "google-api-python-client<3.0.0,>=2.94.0",
     "google-auth<3.0.0,>=2.22.0",
     "argon2-cffi<24.0.0,>=23.1.0",