services: nginx: Replace 'http-port' and 'https-port' with 'listen'.

* doc/guix.texi (Web Services, Version Control Services): Update accordingly.
* gnu/services/certbot.scm (certbot-nginx-server-configurations): Likewise.
* gnu/services/version-control.scm (%cgit-configuration-nginx): Likewise.
* gnu/services/web.scm (<nginx-server-configuration>,
emit-nginx-server-config): Likewise.
* gnu/tests/version-control.scm (%cgit-configuration-nginx,
%git-nginx-configuration): Likewise.
* gnu/tests/web.scm (%nginx-servers, %php-fpm-nginx-server-blocks): Likewise.