Bump library/redis from 6-bullseye to 7-bullseye in /docker (#15712)

Bumps library/redis from 6-bullseye to 7-bullseye.

---
updated-dependencies:
- dependency-name: library/redis
  dependency-type: direct:production
...

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

docker/Dockerfile-workers

@@ -21,7 +21,7 @@ FROM docker.io/library/debian:bullseye-slim AS deps_base
 # which makes it much easier to copy (but we need to make sure we use an image
 # based on the same debian version as the synapse image, to make sure we get
 # the expected version of libc.
-FROM docker.io/library/redis:6-bullseye AS redis_base
+FROM docker.io/library/redis:7-bullseye AS redis_base
 
 # now build the final image, based on the the regular Synapse docker image
 FROM $FROM