Replace deprecated std::aligned_storage usage in Boost.Fiber channels

[knightzone111]

Boost.Fiber currently uses std::aligned_storage in channel iterator storage:

• include/boost/fiber/buffered_channel.hpp
• include/boost/fiber/unbuffered_channel.hpp
• performance/thread/buffered_channel.hpp

std::aligned_storage is deprecated in C++23. This can produce deprecation warnings
with newer compilers, and may become a build failure when projects compile with
-Werror.

The storage is only used as raw storage for placement-new. It can be replaced with
equivalent aligned byte storage:

alignas(alignof(T)) unsigned char storage[sizeof(T)]{};

This keeps the same behavior while avoiding the deprecated C++23 API.

I have a patch ready that replaces the affected std::aligned_storage usages with
alignas(...) unsigned char[...].