Update license on Unicode.org's ConvertUTF code.

The code was relicensed by its owner (Unicode.org) a long time back,
but we still had the old (problematic) license in our fork.

Note that the source files have not been distributed from unicode.org
since 2009 (due to being buggy and unmaintained upstream), but they
were given this license before that.

Fixes https://github.com/llvm/llvm-project/issues/32309

Differential Revision: https://reviews.llvm.org/D66390