Remove Centos-7 due to end of life June 2024

[AverageMarcus]

Change description

Centos-7 has been unsupported / end of life since June 30th 2024. As is no longer a supported OS we also shouldn't support it as a built-in option for image-builder.

Related issues

• N/A

Additional context

References for end of life status:

• https://endoflife.software/operating-systems/linux/centos
• https://mirrors.edge.kernel.org/centos/7/readme

[k8s-ci-robot]

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[AverageMarcus]

/test pull-ova-all

[AverageMarcus]

/test pull-ova-all

[k8s-ci-robot]

@AverageMarcus: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-ova-all	f51e983	link	false	/test pull-ova-all
pull-azure-sigs	f51e983	link	false	/test pull-azure-sigs

Full PR test history. Your PR dashboard. Please help us cut down on flakes by linking to an open issue when you hit one in your PR.

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[AverageMarcus]

/retest-required