ironbank: use ubi8:8.7 in ElasticAgent

[v1v]

The backport didn't work since Elastic Agent is not anymore in 8.x

Proposed commit message

See https://repo1.dso.mil/dsop/elastic/beats/7.x/elastic-agent/-/commit/8d42a44163bd6c38de44565a2d75d016a9ac679f so this should avoid those changes to be done manually by the Ironbank team

Checklist

• 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.next.asciidoc or CHANGELOG-developer.next.asciidoc.

Author's Checklist

• [ ]

How to test this PR locally

Related issues

Use cases

Screenshots

Logs

[elasticmachine]

💚 Build Succeeded

the below badges are clickable and redirect to their specific view in the CI or DOCS

Expand to view the summary

Build stats

• Start Time: 2023-09-19T12:19:27.778+0000

• Duration: 142 min 34 sec

Test stats 🧪

Test	Results
Failed	0
Passed	22415
Skipped	1426
Total	23841

💚 Flaky test report

Tests succeeded.

🤖 GitHub comments

Expand to view the GitHub comments

To re-run your PR in the CI, just comment with:

• /test : Re-trigger the build.

• /package : Generate the packages and run the E2E tests.

• /beats-tester : Run the installation tests with beats-tester.

• run elasticsearch-ci/docs : Re-trigger the docs validation. (use unformatted text in the comment!)