Remove use of std::aligned_storage. NFC (#8942)

This is deprecated in C++23. See
https://github.com/llvm/llvm-project/pull/94169

This also allows us to remove the `/D_ENABLE_EXTENDED_ALIGNED_STORAGE`
from `CMakeLists.txt` which was originally added back in e8ac331501.
Back then we also used in this in `src/mixed_arena.h` but that usage was
removed in #1846.