Add Contao 5 vhost template (fixes #53)

[SaleemAkhter]

Summary

• Adds a Contao 5 vhost template in v2, v2-varnish, and v2-http3 alongside the existing Contao 4 template
• Uses the same nginx configuration as Contao 4 (compatible with Contao 4.8+ Managed Edition and Contao 5) with default PHP 8.3 for new Contao 5 sites
• Keeps the existing Contao 4 template unchanged to avoid breaking current installations

Fixes #53

Test plan

• Import templates via clpctl vhost-templates:import
• Create a Contao 5 site and verify nginx vhost is generated correctly
• Confirm Contao 4 template still works for existing sites