Update confluentinc/cp-schema-registry Docker tag to v8

This MR contains the following updates:

Package	Type	Update	Change
confluentinc/cp-schema-registry	ironbank-docker	major	7.7.2 -> 8.0.0
confluentinc/cp-schema-registry	stage	major	7.7.2 -> 8.0.0

---

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 these updates again.

---

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

---

This MR has been generated by Renovate Bot.