dependabot: add release-note-none label

Followup to https://github.com/openshift/release/pull/28686
in which we ask openshift-ci-bot to enforce a release-note
label on new PRs.

Dependabot PRs do not need release notes. Add a config setting
(copied from cri-o) that tells dependabot to set release-note-none
on new PRs.

Signed-off-by: Ed Santiago <[email protected]>