[8.18] [Security][8.18 & 8.19][Bug] Removes note about suppression and prebuilt rules that's no longer applicable (backport #7081)

[mergify]

Contributes to elastic/docs-content#3290.

Removes note about suppression and prebuilt rules that's no longer applicable. Since 8.18, users can configure suppression on prebuilt rules if they edit them.

Corresponding 9.x and Serverless doc updates are at: elastic/docs-content#3304

Preview

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This is an automatic backport of pull request #7081 done by Mergify.

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