converters/simdutf: update to 8.0.0

One unit test fails as it expects to be run from inside simdutf's
git repository.

The main new feature in this version is that a C API is now provided.

Changelog: https://github.com/simdutf/simdutf/releases/tag/v8.0.0

See also: https://github.com/simdutf/simdutf/issues/921
MFH: no (soname change)