Move available rollbacks under upgrade.rollbacks in checkin request

[pchila]

What does this PR do?

This is a follow-up to PR #11143 moving available rollbacks information under upgrade.rollbacks in the elastic-agent checkin request body.

Why is it important?

To align the structure with the Elasticsearch structure implemented in elastic/fleet-server#5975

Checklist

• I have read and understood the pull request guidelines of this project.
• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in ./changelog/fragments using the changelog tool
• I have added an integration test or an E2E test

Disruptive User Impact

How to test this PR locally

Related issues

Questions to ask yourself

• How are we going to support this in production?
• How are we going to measure its adoption?
• How are we going to debug this?
• What are the metrics I should take care of?
• ...

[elasticmachine]

💔 Build Failed

• Buildkite Build
• Commit: fff34fc

Failed CI Steps

• K8s tests: 1.31.0
• K8s tests: 1.28.9

History

cc @pchila

[elasticmachine]

Pinging @elastic/elastic-agent-control-plane (Team:Elastic-Agent-Control-Plane)

[blakerouse]

lgtm