databases/mysql80-server: fix build with libc++ 19

As noted in the libc++ 19 release notes [1], std::char_traits<> is now
only provided for char, char8_t, char16_t, char32_t and wchar_t, and any
instantiation for other types will fail.

This causes databases/mysql80-client to fail to compile with clang 19
and libc++ 19, resulting in errors similar to:

/usr/include/c++/v1/string:820:42: error: implicit instantiation of

undefined template 'std::char_traits<unsigned char>'

820 |   static_assert(is_same<_CharT, typename

traits_type::char_type>::value,

      |                                          ^
/wrkdirs/usr/ports/databases/mysql80-client/work/mysql-8.0.39/sql/rpl_log_encryption.h:821:14:

note: in instantiation of template class 'std::basic_string<unsigned
char>' requested here

  821 |   Key_string m_encrypted_password;
      |              ^
/usr/include/c++/v1/__fwd/string.h:23:29: note: template is declared

here

23 | struct _LIBCPP_TEMPLATE_VIS char_traits;
   |                             ^

Key_string is defined as std::basic_string<unsigned char>, which is
no longer possible. So redefine it as a std::vector<unsigned char>
instead.

This requires only a few small adjustments in other places: replacing
the length() method with the equivalent size() method, and adjusting
the arguments for the assign() method, which for std::vector takes a
begin and end iterator, instead of a begin iterator and a size.

[1]
https://libcxx.llvm.org/ReleaseNotes/19.html#deprecations-and-removals

PR: 280693
Sponsored by: Netzkommune GmbH