www/caddy: Update to 2.11.6

Changes: https://github.com/caddyserver/caddy/releases/tag/v2.11.6

There are some breaking changes in this release, so you should read
that list. They are primarily security-tightening changes and should
be invisible to most users. If you were relying on uploading files in
headers rather than the body, you'll want to make some changes.