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

@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 bytedeco#753 (comment)

warning: 'char_traits<unsigned short>' is deprecated: char_traits<T> 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]

src/main/java/org/bytedeco/javacpp/tools/Generator.java

@@ -1042,7 +1042,7 @@ boolean classes(boolean handleExceptions, boolean defineAdapters, boolean conver
             out.println("    if (ptr == NULL) {");
             out.println("        return NULL;");
             out.println("    }");
-            out.println("    return JavaCPP_createStringFromUTF16(env, ptr, std::char_traits<unsigned short>::length(ptr));");
+            out.println("    return JavaCPP_createStringFromUTF16(env, ptr, std::char_traits<char16_t>::length(ptr));");
             out.println("}");
             out.println();
         }
@@ -2569,7 +2569,7 @@ void call(MethodInformation methodInfo, String returnPrefix, boolean secondCall)
                 // special considerations for char arrays as strings
                 if (asUtf16(methodInfo, k)) {
                     out.print(indent + "memcpy(");
-                    suffix = ", (std::char_traits<unsigned short>::length(ptr" + k + ") + 1) * sizeof(unsigned short))";
+                    suffix = ", (std::char_traits<char16_t>::length(ptr" + k + ") + 1) * sizeof(unsigned short))";
                 } else {
                     out.print(indent + "strcpy((char*)");
                 }