incus-vm: bump disk default to 50GB, max to 500GB

20GB is insufficient for NixOS builds (Go module cache alone can exceed
it). 50GB is a safer default; 500GB ceiling gives room for heavy workloads.

registry/bpmct/templates/incus-vm/main.tf

@@ -119,13 +119,13 @@ data "coder_parameter" "disk" {
   description  = "Root disk size in GB."
   type         = "number"
   form_type    = "slider"
-  default      = 20
+  default      = 50
   icon         = "/icon/database.svg"
   mutable      = true
   order        = 3
   validation {
     min = 10
-    max = 200
+    max = 500
   }
 }