automation: Update vikunja-api Docker tag to v0.20.4

This MR contains the following updates:

Package	Update	Change
vikunja-api	patch	0.20.2 -> 0.20.4

---

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 this update again.

---

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

---

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