[velero] Update kubectl image repository from bitnami to registry.k8s.io

[SheryarButt]

Special notes for your reviewer:

This PR corresponds to issue #698, and removes the use of bitnami kubectl image in favor of registry.k8s.io

Checklist

[Place an '[x]' (no spaces) in all applicable fields. Please remove unrelated fields.]

• DCO signed
• Chart Version bumped, please refer to the chart version instruction
• Variables are documented in the values.yaml or README.md
• Title of the PR starts with chart name (e.g. [velero])

[judge-red]

I'm not a codeowner for this repo so this isn't a formal review. But this is a breaking change and I think this Helm chart uses semver, so please don't just bump the patch version.

People who use their own registry but don't ovverwrite the tag will have a bad time otherwise.

[mjnagel]

Due to the k8s registry kubectl image not having a shell (and the same is true with most other kubectl images) this work might also require fixing #571 unless there's a viable alternative kubectl image with which, sh, and kubectl.

[SheryarButt]

@mjnagel that was the reason to put this PR to draft, sadly other alternatives are alpine based and doesn't have a sh binary that can be copied over. This would require some rework for the upgrade-crds job. Otherwise, the only viable option is to stick with bitnamilegacy images for now