chore: reduce dependabot PR noise via grouping (#829)

Include sigs.k8s.io/* in the kubernetes group so packages like
controller-runtime and structured-merge-diff land in the existing
weekly kubernetes group PR instead of one PR each.

Prior art:
- https://github.com/kubernetes-sigs/jobset/blob/main/.github/dependabot.yml

Signed-off-by: Kay Yan <kay.yan@daocloud.io>