ci: retire Docker build/push, publishing moved to postguard

[rubenhensen]

Summary

• Removes the build and finalize jobs (the Docker build/push pipeline) from this repo's ci.yml.
• Publishing ghcr.io/encryption4all/cryptify has moved to encryption4all/postguard's own delivery.yml, which builds this crate as a workspace member. Verified working against merge commit 506144c: build-cryptify/scan-cryptify/finalize-cryptify all succeeded and pushed the :edge tag (postguard#293, postguard#309).
• Drops the now-dead parse-release/version/releases_created output plumbing in release-plz-release, whose only consumer was the removed finalize job.
• quality and release-plz-pr/release-plz-release are untouched — out of scope for this ticket.

Part of postguard#293 (step 4 of its checklist). This is the step that stops two pipelines racing on the same GHCR tags — hold off merging until you've had a chance to look at the diff, since it removes this repo's only remaining Docker publish path.

[dobby-coder]

Thanks for the detailed writeup, @rubenhensen — I'm on it. I'll review the diff removing the build/finalize Docker jobs and the release-plz-release output plumbing here in cryptify#206, with an eye on the cutover from postguard#293, and get back to you with a consolidated review.

[dobby-coder]

Heads up — #206 was already merged (2026-08-07 14:00, a361e6a) before I got to review it, with no reviews on record despite the ask to hold off. Since a review can't gate a merged PR, I held off posting one to avoid noise.

Worth flagging: this did remove cryptify's only remaining Docker publish path, now relying entirely on encryption4all/postguard's delivery.yml. If you'd like, I can still review the merged diff after the fact and open a follow-up issue/PR for anything that turns up.