Add Fleet & Agent 9.0.0-beta1 Release Notes

[kilfoyle]

This adds the 9.0.0-beta1 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 preview page
Closes: #1654

Note: Starting with 9.0, the release notes for all products will be combined. That process isn't in place yet, so for this beta release the contents of this PR will be merged into the Installation and Upgrade Guide guide. See the 9.0.0 Beta 1 docs release issue for details.

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[kpollich]

Fleet notes LGTM 🚀

[pierrehilbert]

Thanks @kilfoyle for your work.
Go version has been bumped to 1.22.10 (elastic/elastic-agent@e7328db), this time we bumped .go-version too so it should be the case cc @cmacknz

[kilfoyle]

Thanks @pierrehilbert!
I've changed the Agent Go bump version and PR number to this:
* Update Go version to 1.22.10. {agent-pull}6236[#6236]

[karenzone]

Content LGTM 😄

[kpollich]

Reapproving 🚀

[kilfoyle]

Contents have been copied over into this Stack Docs PR: elastic/stack-docs#2966

[kilfoyle]

Closing. Release Notes have been ported into the Ingest and Upgrade Guide via elastic/stack-docs#2969