unwind the hyper / maghemite work-around

[ahl]

#6706 updated the omicron dependency on maghemite from a pinned revision to (effectively) that pinned version + the hyper v1 migration work from oxidecomputer/maghemite#376. While we would have preferred to simply update the maghemite pinned version to the current main (i.e. inclusive of the hyper v1 update) that is blocked on the work in #6693 due to API incompatibilities introduced by oxidecomputer/maghemite#359 and oxidecomputer/maghemite#360.

Once #6693 is merged, we should update the maghemite dependency to the current maghemite HEAD rev.