net/netdiscover: update to 0.21

While here:

• sort variable order to make portclippy happy
• remove GNU_CONFIGURE_MANPREFIX now defaulted to ${PREFIX}/share

Changelog: https://github.com/netdiscover-scanner/netdiscover/blob/0.21/ChangeLog

PR: 288927
Reported by: Tiago Gasiba <tiago.gasiba@gmail.com> (maintainer)