[#61991106] Rename to walk

Makes more sense to say `vcloud-walk vdcs` than `vcloud-walker vdcs`. Also closer to the Thor command; `thor vcloud_walk:vdcs`

Again - not breaking change as Thor still works. However, this this is worth a small version bump. (Which should have been included in https://github.com/alphagov/vcloud-walker/pull/37/ as that added the functionality.)