www/nginx-devel: update from 1.21.5 to 1.21.6.

<Changelog>

*) Bugfix: when using EPOLLEXCLUSIVE on Linux client connections were

unevenly distributed among worker processes.

*) Bugfix: nginx returned the "Connection: keep-alive" header line in

responses during graceful shutdown of old worker processes.

*) Bugfix: in the "ssl_session_ticket_key" when using TLSv1.3.

</Changelog>