[Security] 8.19.20 release notes

[natasha-moore-elastic]

Summary

Resolves elastic/docs-content#7837: adds the 8.19.20 Security and Endpoint release notes.

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[mergify]

This pull request does not have a backport label. Could you fix it @natasha-moore-elastic? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• v7.x is the label to automatically backport to the 7.x branch.
• v7./d./d is the label to automatically backport to the 7./d branch. /d is the digit

NOTE: backport-skip has been added to this pull request.