[Cases] New cases subfeature privileges

[lcawl]

Description

Relates to elastic/kibana#194898

This PR updates https://www.elastic.co/guide/en/observability/master/grant-cases-access.html to align with the changes in elastic/security-docs#6288

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

Checklist

• Product/Engineering Review
• Writer Review

Follow-up tasks

N/A

[github-actions]

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[alaudazzi]

LGTM