[Synthetics] Revert base image rename from ubuntu to ubi

[emilioalvap]

What does this PR do?

Reverts base image rename introduced in #8078.

Why is it important?

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?
• ...

[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

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[elasticmachine]

💚 Build Succeeded

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• Commit: 02ab881

cc @emilioalvap

[pkoutsovasilis]

ok now I see what happened. The complete container variant for versions >= 9.0.0 is ubi based and unfortunately during the backport mentioned in the PR description this got applied here as well. LGTM thanks for addressing this one @emilioalvap

[elasticmachine]

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