Publish Fleet and Elastic Agent release notes for 8.8.0

[kilfoyle]

Publish 8.8.0 Release Notes:

• Get Fleet content from the Kibana Release Notes PR
• Get Fleet Server content from the 8.8.0 changelog
• Get Elastic Agent content from the 8.8.0 changelog

[kilfoyle]

@pierrehilbert, @cmacknz

I'm still a little fuzzy on the process, but now that the changelog tool supports Fleet Server (via Michel's PR) should I wait for the release point person to generate the changelog for both Agent and Fleet Server?

Also, would it perhaps be possible in each release for the changelogs to be generated close to feature freeze, or after BC1? Most docs teams try to help the Docs point person by having the Release Notes PRs up and ready a week or two before GA. For example, in the "Confirm release notes" section of the current Docs release PR, I see that the Elasticsearch, Kibana, and Logstash teams already have their 8.8.0 release notes merged or ready to merge. If there's nothing preventing us, I think it would ideal if we could align with that timing.