[k8s] Correct agent image repository

[pkoutsovasilis]

What does this PR do?

This PR updates the registry of the elastic-agent container image to reflect the latest and correct one

Why is it important?

It is mandatory as elastic-agent container image has migrated to docker.elastic.co/elastic-agent/

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

How to test this PR locally

Follow either one of the elastic-agent Helm chart examples

Related issues

Closes #6725

[pchila]

Looks good, there a few more references to the old docker image name in /pkg/testing/kubernetes/supported.go that I think need to be updated.

I also don't think this needs to be backported to 8.* versions as we stopped publishing elastic-agent image under docker.elastic.co/beats/ starting from 9.0 version

[elastic-sonarqube]

Quality Gate passed

Issues
0 New issues
0 Fixed issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
0.0% Duplication on New Code

See analysis details on SonarQube