x11-fonts/gentium-book: New port

This is the other half of the Gentium-7.000 updates. The former
'Gentium Plus Book' has now become 'Gentium Book' and is distributed
as a separate font to the original Gentium.

See https://software.sil.org/gentium/history/ and
https://software.sil.org/gentium/download/

Changes: https://software.sil.org/gentium/v7changes/
Reported by: repology