[Fiber] aligned_storage is deprecated

From: Niall Douglas (s_sourceforge_at_[hidden])
Date: 2023-12-12 16:32:30

On Boost 1.81 with clang 18 in C++ 23 mode:

```
[build] /usr/include/boost/fiber/future/detail/shared_state.hpp:160:19: 
warning: 'aligned_storage<4, 4>' is deprecated [-Wdeprecated-declarations]
[build]   160 |     typename std::aligned_storage< sizeof( R), alignof( 
R) >::type  storage_{};
[build]       |                   ^
[build] /usr/include/boost/fiber/future/promise.hpp:28:22: note: in 
instantiation of template class 
'boost::fibers::detail::shared_state<int>' requested here
[build]    28 |     typedef typename shared_state< R >::ptr_type   ptr_type;
```

It could have been fixed upstream since, I haven't checked.

Niall