Add Fleet & Agent 8.15.4 Release Notes

[kilfoyle]

This adds the 8.15.4 Fleet & Elastic Agent Release Notes:

• No Fleet contents in Kibana Release Notes PR
• No Fleet Server contents in BC1 changelog
• No Elastic Agent contents in BC1 changelog

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[florent-leborgne]

@kilfoyle just a note that for Kibana there's currently a bit of uncertainty due to PRs in the Kibana repo not being reindexed since the issues from last week, which causes the release notes automation and manual search for PRs to not retrieve anything.

[kilfoyle]

Ah, good to know! Thanks for the heads up @florent-leborgne!

[karenzone]

LGTM

[karenzone]

@florent-leborgne, thanks for the info. What's the impact on final release notes?

[pierrehilbert]

I think we should be good with merging this one today

[pierrehilbert]

We have one bug fix that should be in:
elastic/elastic-agent@0f22fd2

[karenzone]

@pierrehilbert, I'm lacking context here. Can I get more info about what you want to show in release notes?
A github suggestion would be helpful. Or we can discuss.

UPDATE: I'll take a shot at it, and we can go from there.

[leemthompo]

approving to unblock release

[karenzone]

LGTM

[karenzone]

@leemthompo, I pushed the last change and can't do final sign off. Reapprove please?