[Observability][Cases][BUG]: Case templates were GA'd in 8.18

[nastasha-solomon]

Description

Documentation sets edited in this PR

Check all that apply.

• Stateful (docs/en/observability/*)
• Serverless (docs/en/serverless/*)
• Integrations Developer Guide (docs/en/integrations/*)
• None of the above

Related issue

Closes # elastic/docs-content#5209

Checklist

• Product/Engineering Review
• Writer Review

Follow-up tasks

Select one.

• This PR does not need to be ported to another doc set because:
  • The concepts in this PR only apply to one doc set (serverless or stateful)
  • The PR contains edits to both doc sets (serverless and stateful)
• This PR needs to be ported to another doc set:
  • Port to stateful docs: [Internal]: Case templates were GA'd in 8.18 docs-content#5210
  • Port to serverless docs: [Internal]: Case templates were GA'd in 8.18 docs-content#5210

[github-actions]

🤖 GitHub comments

Just comment with:

• run docs-build : Re-trigger the docs validation. (use unformatted text in the comment!)

[mdbirnstiehl]

🐦