docs: update etcd kvstore migration instructions
docs: update etcd kvstore migration instructions

The Helm chart does not have a "global" section for etcd.
This also updates the pod name to cilium-preflight-check-1234
and adds a way to confirm a successful migration.

Signed-off-by: hhoover <00083a6e13bccdb336bccc13c4a6a13ddced3cf2@isovalent.com>

cilium/cilium,cilium/cilium,cilium/cilium,michi-covalent/cilium,michi-covalent/cilium,michi-covalent/cilium,cilium/cilium,cilium/cilium,michi-covalent/cilium,michi-covalent/cilium