[bug] Prevent changing subnet and IP objects assigned to a VPN Server

Description

There is a need to prevent changes to the subnet and IPAdress objects when they are assigned to a VPN server. Modifying the subnet of the VPN when there are existing VPN clients creates collisions in provisioned IPs, leading to potential network issues.

Steps To Reproduce
Steps to reproduce the behavior:

1. Create a WireGuard VPN server and assign a subnet and IP to it.
2. Create a template for this VPN and assign it to a device
3. Edit the subnet object created in step 1 and change the subnet field
4. Assign the VPN template to another device

Expected behavior
Prevent changing subnet and IP address objects if there are existing VpnClients

Actual Behavior
Both devices get IP address from different subnets

The second peer has an IP address from

System Information:

• OS: Ubuntu 24.04 LTS
• Python Version: Python 3.11.2
• Django Version: Django 5.2.x