Update hmctspublic.azurecr.io/base/java Docker tag to v21

[renovate]

This PR contains the following updates:

Package	Type	Update	Change
hmctspublic.azurecr.io/base/java	final	major	17-distroless -> 21-distroless

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Configuration

📅 Schedule: Branch creation - "after 7am and before 11am every weekday" in timezone Europe/London, Automerge - At any time (no schedule defined).

🚦 Automerge: Disabled by config. Please merge this manually once you are satisfied.

♻ Rebasing: Whenever PR becomes conflicted, or you tick the rebase/retry checkbox.

🔕 Ignore: Close this PR and you won't be reminded about this update again.

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

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[renovate]

Renovate Ignore Notification

Because you closed this PR without merging, Renovate will ignore this update. You will not get PRs for any future 21.x releases. But if you manually upgrade to 21.x then Renovate will re-enable minor and patch updates automatically.

If you accidentally closed this PR, or if you changed your mind: rename this PR to get a fresh replacement PR.