[8.18] [8.18] New sub-feature priv for case assignees (backport #6641)

[mergify]

Partially addresses elastic/docs-content#333. Makes minor changes to future-proof docs for giving full access to manage cases and settings.

Preview: Cases requirements

• 9.0 and Serverless docs (all): [9.0 & Serverless] New sub-feature priv for case assignees  docs-content#831
• Kibana docs: [DOCS][8.18] New sub-feature priv for case assignees kibana#215162
• Observability docs: [8.18] New sub-feature priv for case assignees observability-docs#4850

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  This is an automatic backport of pull request [8.18] New sub-feature priv for case assignees #6641 done by Mergify.

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