docs(self-managed): mention the change of using image.tag

[aabouzaid]

Description

Task: https://github.com/camunda/camunda-platform-helm/issues/2067

When should this change go live?

• [x] This is a bug fix, security concern, or something that needs urgent release support.
• [ ] This is already available but undocumented and should be released within a week.
• [ ] This on a specific schedule and the assignee will coordinate a release with the DevEx team. (apply hold label or convert to draft PR)
• [ ] This is part of a scheduled alpha or minor. (apply alpha or minor label)
• [ ] There is no urgency with this change and can be released at any time.

PR Checklist

• [x] My changes are for an already released minor and are in /versioned_docs directory.
• [x] My changes are for the next minor and are in /docs directory (aka /next/).

• [ ] My changes require an Engineering review, and I've assigned the Engineering DRI or delegate.
• [x] My changes require a technical writer review, and I've assigned @camunda/tech-writers as a reviewer.

[aabouzaid]

@conceptualshark FYI, I just made a small change and added a deprecation note because that key will be actually removed as it's confusing and not needed anymore.

[akeller]

Merging - this will get released later today with the alpha3 changes.