www/xcaddy: update to 0.4.4, modernize port, add security warning

• Convert from GH_TUPLE to GO_MODULE for cleaner dependency management
• Use USES=go:modules,run to properly handle runtime Go dependency
• Add pkg-message warning about security implications of running Caddy servers as root, with instructions for secure deployment
• Remove PORTREVISION (reset on version update)

xcaddy builds custom Caddy web servers which often need to bind to
privileged ports. Users should be aware of the security implications
and run their custom Caddy builds as unprivileged users with portacl-rc
for port binding capabilities.

PR: 279391
Reported by: Robert William Vesterman