chore(deps): update quay.io/shivering-isles/dovecot docker tag to v2

This MR contains the following updates:

Package	Update	Change
quay.io/shivering-isles/dovecot (source)	major	0.4.1 -> 2.3.21

---

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 SI Renovate Bot repository.