github: enable dependabot

[pohly]

What type of PR is this? /kind cleanup

What this PR does / why we need it:

After the trial in csi-test and csi-driver-iscsi we want to role out the same automatic dependency update also in other repos.

Does this PR introduce a user-facing change?:

NONE

[xing-yang]

/lgtm /approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: pohly, xing-yang

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