Update mitre/heimdall2 Docker tag to v2.6.15

This MR contains the following updates:

Package	Type	Update	Change
mitre/heimdall2	ironbank-docker	patch	2.6.14 -> 2.6.15
mitre/heimdall2		patch	2.6.14 -> 2.6.15
mitre/heimdall2	stage	patch	2.6.14 -> 2.6.15

---

Configuration

📅 Schedule: At any time (no schedule defined).

🚦 Automerge: Disabled due to failing status checks.

♻ 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, click this checkbox.

---

This MR has been generated by Renovate Bot.