centos-7,centos-stream8: Mark as deprecated with recent EOLs

[0xFelix]

What this PR does / why we need it:

Mark CentOS 7 and Stream 8 as deprecated with recent EOLs

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

Special notes for your reviewer:

Release note:

CentOS 7 and Stream 8 templates are now deprecated

[0xFelix]

/cherry-pick release-v0.29

[kubevirt-bot]

@0xFelix: once the present PR merges, I will cherry-pick it on top of release-v0.29 in a new PR and assign it to you.

In response to this:

/cherry-pick release-v0.29

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[dominikholler]

FYI @acardace @metalice

[ksimon1]

/lgtm

[ksimon1]

/retest

[ksimon1]

/approve
@0xFelix do we want to keep the testing automation for these templates in place?

[kubevirt-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ksimon1

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[0xFelix]

We can drop the automation once we remove the templates.

[kubevirt-bot]

@0xFelix: new pull request created: #599

In response to this:

/cherry-pick release-v0.29

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes-sigs/prow repository.