Bump centos from 7 to 8 in /tekton/images/openssh-server

[dependabot]

Bumps centos from 7 to 8.

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[tekton-robot]

Hi @dependabot[bot]. Thanks for your PR.

I'm waiting for a tektoncd member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[tekton-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: vdemeester

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[tekton-robot]

The following Tekton test failed:

Test name	Commit	Details	Required	Rerun command
plumbing-image-build	687b051	link	true	/test plumbing-image-build

[tekton-robot]

Issues go stale after 90d of inactivity.
Mark the issue as fresh with /remove-lifecycle stale with a justification.
Stale issues rot after an additional 30d of inactivity and eventually close.
If this issue is safe to close now please do so with /close with a justification.
If this issue should be exempted, mark the issue as frozen with /lifecycle frozen with a justification.

/lifecycle stale

Send feedback to tektoncd/plumbing.

[dependabot]

Superseded by #1685.