Update istio/istioctl Docker tag to v1.19.4

This MR contains the following updates:

Package	Type	Update	Change
istio/istioctl	ironbank-docker	patch	1.19.3-distroless -> 1.19.4-distroless
istio/istioctl		patch	1.19.3 -> 1.19.4
istio/istioctl	stage	patch	1.19.3-distroless -> 1.19.4-distroless

---

⚠ Dependency Lookup Warnings ⚠

Warnings were logged while processing this repo. Please check the logs for more information.

---

Configuration

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

🚦 Automerge: Enabled.

♻ 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.