Modernize NSG filtering tutorial with Linux VMs and SSH keys

- Replace Windows Server VMs with Ubuntu 24.04 LTS
- Migrate from password authentication to SSH key authentication
- Auto-generate SSH keys in all methods (Portal, PowerShell, CLI)
- Replace RDP (port 3389) with HTTPS (port 443) for management VM
- Replace IIS with nginx web servers
- Configure vm-web with HTTP (port 80) only
- Configure vm-mgmt with HTTPS (port 443) using self-signed certificates
- Install web servers via VM Run Command instead of RDP sessions
- Update all security warnings and best practices
- Demonstrate NSG filtering with opposite port access patterns
- Remove all Windows/RDP/password references

This update improves security by using modern Linux practices,
SSH key authentication, and eliminates deprecated RDP exposure.

Author: asudbring