[8.x] Add Fleet & Agent 8.15.4 Release Notes (backport #1417)

[mergify[bot]]

This adds the 8.15.4 Fleet & Elastic Agent Release Notes:

• No Fleet contents in Kibana Release Notes PR
• No Fleet Server contents in BC1 changelog
• No Elastic Agent contents in BC1 changelog

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This is an automatic backport of pull request #1417 done by Mergify.

[github-actions[bot]]

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[karenzone]

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