Update redis Docker tag to v8

This MR contains the following updates:

Package	Update	Change
redis	major	7.2.4 -> 8.0.2
redis	major	7-alpine -> 8-alpine

---

Configuration

Schedule: Branch creation - At any time (no schedule defined), Automerge - At any time (no schedule defined).

Automerge: Disabled by config. Please merge this manually once you are satisfied.

Rebasing: Whenever MR becomes conflicted, or you tick the rebase/retry checkbox.

Ignore: Close this MR and you won't be reminded about these updates again.

---

• If you want to rebase/retry this MR, check this box

---

This MR has been generated by Renovate Bot.

testserver/docker_mail/templates/docker-compose.yml

@@ -2,7 +2,7 @@ services:
 
   # External dependencies
   redis:
-    image: redis:7-alpine
+    image: redis:8-alpine
     restart: always
     volumes:
       - "{{ basedir }}/mailu/redis:/data"