Update istio/node-agent-k8s Docker tag to v1.5.6

This MR contains the following updates:

Package	Type	Update	Change
istio/node-agent-k8s	ironbank-docker	patch	1.5.4 -> 1.5.6
istio/node-agent-k8s		patch	1.5.4 -> 1.5.6
istio/node-agent-k8s	stage	patch	1.5.4 -> 1.5.6

---

Renovate configuration

📅 Schedule: 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.