Update gradle Docker tag to v6.9.0

This MR contains the following updates:

Package	Type	Update	Change
gradle	image	minor	6.0.1-jdk11 -> 6.9.0-jdk11

---

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 this update again.

---

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

---

This MR has been generated by Renovate Bot.