Change k/k8s.io branch to "main" in image promotion PRs

[puerco]

What type of PR is this?

/kind bug

What this PR does / why we need it:

During the 1.21.0-alpha.3 release cut the automatic PR for image promotion failed because the default branch in kubernetes/k8s.io was renamed to "main" and we had not updated the name in krel. This PR fixes it.

Which issue(s) this PR fixes:

Slack Ref: https://kubernetes.slack.com/archives/CJH2GBF7Y/p1612874602190300?thread_ts=1612863244.184400&cid=CJH2GBF7Y

Special notes for your reviewer:

Does this PR introduce a user-facing change?

- Image promotion pull requests are now created against `main` in `kubernetes/k8s.io` as the default branch has been renamed,

Signed-off-by: Adolfo García Veytia (Puerco) adolfo.garcia@uservers.net

[saschagrunert]

/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: puerco, saschagrunert

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• cmd/krel/OWNERS [puerco,saschagrunert]

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