ui: sdn prefix-lists: change prefix format to allow bigger subnets

Change the format on the prefix-list prefix input field. This allows us
to use bigger CIDRs such as /0.

Signed-off-by: Gabriel Goller <g.goller@proxmox.com>
Link: https://lore.proxmox.com/20260513083430.63529-4-g.goller@proxmox.com
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>