rgw: avoid using std::aligned_storage_t

rgw: avoid using std::aligned_storage_t

std::aligned_storage_t was deprecated in C++23, to be prepared for it,
let's use alignas for the same behavior. because the 3 * 8 (with LP64
data model) is not power-of-2, while `alignas()` requires an alignment
of power of 2. so we use `std::bit_ceil()` to calculate the minimum
alignment greater or equal to this number.

Signed-off-by: Kefu Chai <tchaikov@gmail.com>