databases/kbibtex: Bump C++ version to C++17

Backport upstream commit 4d9ce600fdd1deb035a26e4f7922d35b2f41beaf
Fixes build with ICU 76+

Reference:
https://github.com/KDE/kbibtex/commit/4d9ce600fdd1deb035a26e4f7922d35b2f41beaf

PR: 278420
Approved by: portmgr (blanket, build fix)