Revert elastic-agent 9.0.0 pinning

[pchila]

In PR #6682 we needed to pin elastic agent package version to 9.0.0 due to issues with calculating previous minor version after bumping agent version to 9.1.0 (see CI runs for PR #6649).

The problem comes from the fact that 9.0.0 version is not yet released and that there are not 2 minor versions in the 9.x major version.

The goal of this issue is to revert the hack introduced in PR #6682:

• either waiting for 9.0.0 to be released (more realistically we would need also 9.1.0 released)
• or changing the way previous minor versions are retrieved allowing for some slack when missing versions.

[elasticmachine]

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