www/caddy: Update to 2.6.0

Also, improve the rc(8) script with updates by scf, allowing proper
daemon shutdown regardless of whether the admin API is enabled.

2.6.0 is a significant update, bringing automatic HTTP/3 (QUIC) support,
a fully abstracted fileserver (serve static files from a database, a
tarball, a script, etc.), and lots more.

Extensive update notes: https://github.com/caddyserver/caddy/releases/tag/v2.6.0