devel/simdjson: update to 4.6.8
A bug fix release.
Changelog: https://github.com/simdjson/simdjson/releases/tag/v4.6.7
Changelog: https://github.com/simdjson/simdjson/releases/tag/v4.6.8
MFH: 2026Q3
(cherry picked from commit 537a529267085d40a342044bd1251c423ac55aed)