[8.15] Add Fleet & Agent 8.15.5 Release Notes (backport #1496)

[mergify[bot]]

This adds the 8.15.5 Fleet & Elastic Agent Release Notes:

• No Fleet contents in Kibana Release Notes PR
• No Fleet Server contents in BC2 changelog
• Elastic Agent contents in BC2 changelog

Closes: https://github.com/elastic/ingest-docs/issues/1497

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This is an automatic backport of pull request #1496 done by Mergify.

[github-actions[bot]]

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