Remove redundant incus_ prefix from root module variables

Rename variables to remove the useless incus_ prefix:
- incus_hypervisors → hypervisors
- incus_witnesses → witnesses
- incus_token → token
- incus_networks → networks
- incus_storage_pools → storage_pools

Updates all references across provider.tf, groups.tf, network.tf,
storage.tf, and .auto.tfvars files.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

incus/storage.auto.tfvars

@@ -1,4 +1,4 @@
-incus_storage_pools = {
+storage_pools = {
   "san" = {
     driver            = "lvmcluster"
     hypervisor_config = { "lvm.vg_name" = "nucVG" }