ci: unpin agent version

[pkoutsovasilis]

What does this PR do?

This reverts the elastic-agent version pinning in the CI introduced by this PR

Why is it important?

Because we need to run our CI with the latest and greatest 🙂

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

None this regards solely the elastic-agent CI

How to test this PR locally

Open a draft PR 🙂

Related issues

N/A

[elastic-sonarqube]

Quality Gate passed

Issues
0 New issues
0 Fixed issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
No data about Duplication

See analysis details on SonarQube

[elasticmachine]

💚 Build Succeeded

• Buildkite Build
• Commit: b7724c1

cc @pkoutsovasilis

[elasticmachine]

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

[pchila]

LGTM