Add workflow to validate UBI images pass RedHat Certification

[shaun-nx]

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Proposed changes

This change adds a new workflow file, openshift-certification.yml which is responsible for validating that our UBI based images of NGF, NGINX OSS, and NGF Operator will pass the RedHad certification process.

This workflow uses RedHat Openshift Preflight tool to pre-certify images before they are published: https://github.com/redhat-openshift-ecosystem/openshift-preflight

Closes https://github.com/nginx/nginx-gateway-fabric/issues/3909

Checklist

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• [ ] I have read the CONTRIBUTING doc
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] I have checked that all unit tests pass after adding my changes
• [ ] I have updated necessary documentation
• [ ] I have rebased my branch onto main
• [ ] I will ensure my PR is targeting the main branch and pulling from my branch from my own fork

Release notes

If this PR introduces a change that affects users and needs to be mentioned in the release notes, please add a brief note that summarizes the change.