rpk: expose the stretch cluster dashboard as --dashboard operations-stretch

Per review feedback, drop the --cluster-type flag and the dashboard-set
overlay: a separate flag suggested every dashboard had a stretch
variant, when only one stretch dashboard exists. The stretch cluster
dashboard is now a regular dashboard map entry selected with
--dashboard operations-stretch, so it shows up in --dashboard help and
shell completion like every other dashboard and follows the same
download-with-embedded-fallback flow.

Ref K8S-855

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>