converters/simdutf: update to 6.4.2

Changelog: https://github.com/simdutf/simdutf/releases/tag/v6.4.1
Changelog: https://github.com/simdutf/simdutf/releases/tag/v6.4.2

MFH: 2025Q2
(cherry picked from commit 766d1cf0704033a3c71cbfeecd141d02aad1f2a6)