www/caddy: Update to 2.3.0 and take maintainership

Thanks to Daniel Tihanyi for all his work maintaining this port!

While here, a number of cleanups:

• Simplify Makefile
• Remove pkg-message whose instructions were superseded by @sample
• Re-enable cgo. It seems to work fine now.
• Rename the rc(8) 'validate' subcommand to 'configtest' for clarity and consistency
• Add a little more info to the sample Caddyfile
• Point to instructions for getting access logs working

Changes: https://github.com/caddyserver/caddy/releases/tag/v2.3.0

PR: 254436
Approved by: Daniel Tihanyi (outgoing maintainer)