[8.16] Add Fleet & Agent 8.16.0 Release Notes (backport #1412)

[mergify[bot]]

This adds the 8.16.0 Fleet & Elastic Agent Release Notes:

• Fleet contents from Kibana Release Notes PR
• Fleet Server contents from BC3 changelog
• Elastic Agent contents from BC3 changelog

See DOCS PREVIEW

Closes: https://github.com/elastic/ingest-docs/issues/1411

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This is an automatic backport of pull request #1412 done by Mergify.

[github-actions[bot]]

A documentation preview will be available soon.

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