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/groups.tf

@@ -4,7 +4,7 @@ resource "incus_cluster_group" "default" {
 }
 
 resource "incus_cluster_group_member" "hypervisor" {
-  for_each      = var.incus_hypervisors
+  for_each      = var.hypervisors
   cluster_group = incus_cluster_group.default.name
   member        = each.key
 }
@@ -15,7 +15,7 @@ resource "incus_cluster_group" "witness" {
 }
 
 resource "incus_cluster_group_member" "witness" {
-  for_each      = var.incus_witnesses
+  for_each      = var.witnesses
   cluster_group = incus_cluster_group.witness.name
   member        = each.key
 }