x11-fonts/gentium-basic: deprecate and mark for expiry

Mark this port for expiry at the end of Q3

The Gentium font family has been rationalized with the release of
Gentium 7.000 and Gentium Book 7.000, and users should switch to one
of those packages. Older variants of the font are no longer
recommended by upstream. See
https://software.sil.org/gentium/download/ for details.