feat(modeler/element-templates): add details for deprecated property

[nikku]

Description

Our existing documentation on deprecating element templates is insufficient as highlighted in this conversation.

This PR improves the documentation, ensures that the recommended deprecation pattern is highlighted first. It also aligns the section with other sections in the document and links upstream documentation.

When should this change go live?

• This is a bug fix, security concern, or something that needs urgent release support. (add bug or support label)
• This is already available but undocumented and should be released within a week. (add available & undocumented label)
• This is on a specific schedule and the assignee will coordinate a release with the Documentation team. (create draft PR and/or add hold label)
• This is part of a scheduled alpha or minor. (add alpha or minor label)
• There is no urgency with this change (add low prio label)

PR Checklist

• The commit history of this PR is cleaned up, using {type}(scope): {description} commit message(s)

• My changes are for an upcoming minor release and are in the /docs directory (version 8.8).
• My changes are for an already released minor and are in a /versioned_docs directory.

• I added a DRI, team, or delegate as a reviewer for technical accuracy and grammar/style:
  • Engineering team review
  • Technical writer review via @camunda/tech-writers unless working with an embedded writer.

[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.

[nikku]

Backported to 8.8, deprecated is undocumented in <= 8.7 (ref).

[lmbateman]

Looks good! Made a few minor suggestions, but none are essential.

[lmbateman]

Thanks for the changes! Just two minor typos; otherwise, looks great.

[lmbateman]

Two minor proofreading suggestions. Approving in advance for expediency.

[nikku]

Should be all clear now. @camunda/tech-writers looking for your ✔️