Update ghcr.io/cloudnative-pg/postgresql Docker tag to v18

[renovate]

This PR contains the following updates:

Package	Update	Change
ghcr.io/cloudnative-pg/postgresql	major	16 → 18
ghcr.io/cloudnative-pg/postgresql	major	17 → 18

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Configuration

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

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

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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 18.x releases. But if you manually upgrade to 18.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.