Add Fleet and Agent release notes for 8.19.20

[vishaangelova]

This PR adds the 8.19.20 release notes for Fleet and Elastic Agent. The content is sourced from these generated changelogs:

• Add Elastic Agent 8.19.20 release notes elastic-agent#16085
• Add Fleet Server 8.19.20 release notes fleet-server#7591

No additional forward- or backports are required.
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