renovate: Update vikunja-redis Docker tag to v6.2.6

This MR contains the following updates:

Package	Update	Change
vikunja-redis	patch	6.2.5-alpine3.14 -> 6.2.6-alpine3.14

---

Configuration

📅 Schedule: At any time (no schedule defined).

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

♻ Rebasing: Whenever MR is behind base branch, or you tick the rebase/retry checkbox.

🔕 Ignore: Close this MR and you won't be reminded about this update again.

---

This MR has been generated by Renovate Bot. The local configuration can be found in the local Renovate Bot repository.