Publish Fleet and Elastic Agent release notes for 8.7.0

[dedemorton]

The process for adding Fleet and Elastic Agent release notes is still in flux. See #124.

But the 8.7.0 release is rapidly approaching. We need to:

• Check in on the current status of the release notes script.
• Decide what format we want to provide for 8.7.0. (For example, do we want to continue duplicating the Fleet release notes, or just point users to the Kibana release notes for Fleet-related items. We will need to publish release notes for Elastic Agent and Fleet Server in some format somewhere.)
• Make sure the release notes get published for 8.7.0.

Contacts/stakeholders:

@pierrehilbert
@cmacknz

cc @kilfoyle

[pierrehilbert]

Adding @jen-huang and @kpollich for the Fleet part.
I would like to spend time to have everything using the new fragment system (not the case for Beats yet) and to have a new pipeline that will remove some manual action here.
However, I'm not sure it will be ready for the 8.7 release.

[kilfoyle]

@endorama or @cmacknz Would you be the right folks to ask about running the elastic-agent-changelog-tool? If you don't mind running that I'll add the output into the Release Notes PR.

[cmacknz]

Here's the YAML changelog summary generated from each of the changelog fragments in the agent repository: elastic/elastic-agent#2395