[Docs] Adds 8.16.0 release notes (elastic#198166)

This PR adds release notes for Kibana 8.16.0

It also adds new deprecations to the Upgrade notes file.

There are many entries so here's a list of the sections that have been
edited and are ready for review so far:

- [x] AGPL license
- [x] Alerting @lcawl 
- [x] Dashboards and visualizations
- [x] Data ingestion and Fleet @kilfoyle 
- [x] Discover
- [x] Elastic Observability solution @bmorelli25 
- [x] Elastic Search solution @leemthompo 
- [x] Elastic Security solution
- [x] Kibana platform
- [x] Kibana security
- [x] Machine Learning @szabosteve @leemthompo 
- [x] Management

Closes: elastic/platform-docs-team#502

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Co-authored-by: István Zoltán Szabó <[email protected]>
Co-authored-by: David Kilfoyle <[email protected]>
Co-authored-by: Liam Thompson <[email protected]>
Co-authored-by: Giorgos Bamparopoulos <[email protected]>
Co-authored-by: Brandon Morelli <[email protected]>
Co-authored-by: lcawl <[email protected]>