[8.9] [Enhancement][ESS] Only open or acknowledged alerts are considered for alert suppression (backport #5122)

[mergify]

Contributes to #4916

Suppress detection alerts | Confirm suppressed alerts: Added note about suppression only applying to open or acknowledged alerts.

Twin Serverless PR: https://github.com/elastic/staging-serverless-security-docs/pull/334

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This is an automatic backport of pull request #5122 done by Mergify.

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