fix: deprecated use of std::char_traits<unsigned short>::length

[Clement-Devevey]

Fix the warning about deprecated use of char_traits by char_traits<char16_t> as it has been discussing in this thread:
@codeinred Since we've decided to make C++11 a requirement for future versions of JavaCPP, let's just replace unsigned short with char16_t. Thanks!

Originally posted by @saudet in #753 (comment)

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Original issue is;
warning: 'char_traits' is deprecated: char_traits for T not equal to char, wchar_t, char8_t, char16_t or char32_t is non-standard and is provided for a temporary period. It will be removed in LLVM 19, so please migrate off of it. [-Wdeprecated-declarations]

[Clement-Devevey]

Having one issue about doing this:
in JavaCPP_createStringFromUTF16 the return env->NewString(ptr, length); isn't working:
Cannot initialize a parameter of type 'const jchar *' (aka 'const unsigned short *') with an lvalue of type 'const char16_t *'