The shipped catch-all default site uses `ssl_reject_handshake`, which is unsupported on nginx older than 1.19.4 and causes nginx to fail to start on Debian Bullseye. The postinst now detects the installed nginx version and installs an HTTP-only variant of the catch-all on older systems.
Renaming `/etc/nginx/sites-enabled/default` to `.dokku-disabled` left a broken symlink that nginx's `include /etc/nginx/sites-enabled/*;` glob still picked up, breaking config validation. Files inside `sites-enabled/` are now removed; the real config in `sites-available/default` is still preserved via rename.
Fresh apt installs now drop a catch-all server block at `/etc/nginx/conf.d/00-default-vhost.conf` that uses `ssl_reject_handshake on` and `return 444` to drop requests with unknown Host headers. Conflicting upstream nginx default vhosts are renamed to `*.dokku-disabled` rather than deleted, preserving any local edits. The new `dokku/install_default_site` debconf flag opts out of the install. Upgrades leave existing nginx config untouched.
