fix: Core() is deprecated use CoreV1() instead.

[oilbeater]

Replace Core() as it is deprecated.

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[k8s-reviewable]

This change is 

[coveralls]

Coverage decreased (-0.04%) to 36.765% when pulling 2c965e290740672da13568fe6d0ebdda948e928c on oilbeater:fix/remove_deprecated into 94487ca on kubernetes:master.

[coveralls]

Coverage remained the same at 36.803% when pulling b3cec74 on oilbeater:fix/remove_deprecated into 94487ca on kubernetes:master.

[aledbf]

/lgtm

[aledbf]

@oilbeater thanks!