Quick overhaul of outdated information in the migration guide

[berndruecker]

Description

Quick updates of updated information about migration and timelines.
Quick fix for now - planing a proper overhaul of the migration guide later this year when we release more tooling support

When should this change go live?

• 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

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

[github-actions]

👋 🤖 ✅ Looks like the changes were ported across versions, nice job! 🎉

You can read more about the versioning within our docs in our documentation guidelines.

[berndruecker]

Not sure about brnaching - but changes can go live in all versions of the doc :-)

[christinaausley]

Backported and approved -- nice writing!

[akeller]

Backported and approved -- nice writing!

Will do a handwavy approval to get this merged.