Update redis requirement from <6,>=5.0 to >=5.0,<8 in /backend

Updates the requirements on [redis](https://github.com/redis/redis-py) to permit the latest version.
- [Release notes](https://github.com/redis/redis-py/releases)
- [Changelog](https://github.com/redis/redis-py/blob/master/CHANGES)
- [Commits](redis/redis-py@v5.0.0...v7.1.1)

---
updated-dependencies:
- dependency-name: redis
  dependency-version: 7.1.1
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

backend/pyproject.toml

@@ -26,7 +26,7 @@ dependencies = [
     "whitenoise>=6.8,<7",
     "django-environ>=0.11,<1",
     "psycopg[binary]>=3.1,<4",
-    "redis>=5.0,<6",
+    "redis>=5.0,<8",
     "channels>=4.1,<5",
     "channels-redis>=4.2,<5",
     "daphne>=4.1,<5",