databases/kdb: Fix build with ICU 75+

ICU 75+ requires at least c++17 for c++; adapt.

While here: Add LICENSE info, clean up whitespace, pet portclippy(1),
and remove false hope from 7 years ago that sla...ahem...child ports will
ever be a thing.

PR: 278420, 283577
Reported by: jbeich, diizzy