[8.18] [Release notes][8.18.x] Add known issue to release notes for "Entity Risk Score documents eventually fail to persist (backport #6866)

[mergify]

Contributes to elastic/docs-content#1548 by adding a known issue about the risk score feature in 8.18.x.

Preview:

• 8.18.0
• 8.18.1
• 8.18.2

Corresponding PRs

• 9.x/Serverless: [Release notes][Serverless & Sec 9.x] Add known issue to release notes for "Entity Risk Score documents eventually fail to persist docs-content#1550

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  This is an automatic backport of pull request [Release notes][8.18.x] Add known issue to release notes for "Entity Risk Score documents eventually fail to persist  #6866 done by Mergify.

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