[9.0](backport #45251) Update user agent

[mergify]

Proposed commit message

Update default user agent to distinguish between all beat modes:

• Standalone indicates that the beat is not running under agent.
• Unmanaged indicates that the beat is running under agent but not managed by Fleet.
• Managed indicates that the beat is running under agent and managed by Fleet.

privilege mode information is only exposed under agent

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

Disruptive User Impact

Users relying on specific user agents could be impacted. We've made no guarantees of stability for those and the only parts that changes are in the comments section.

Author's Checklist

• verify user agent from a standalone agent
• verify user agent from an unmanaged agent
• verify user agent from a managed agent

---

This is an automatic backport of pull request #45251 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 2a99bfa has failed:

On branch mergify/bp/9.0/pr-45251
Your branch is up to date with 'origin/9.0'.

You are currently cherry-picking commit 2a99bfaa5.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   CHANGELOG.next.asciidoc
	modified:   libbeat/beat/beat.go
	modified:   libbeat/beat/beat_test.go

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   NOTICE.txt
	both modified:   go.mod
	both modified:   go.sum

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[elasticmachine]

Pinging @elastic/elastic-agent-data-plane (Team:Elastic-Agent-Data-Plane)