osd: Replace deprecated std::align_storage_t with alignas

C++23 has been enabled, causing deprecated warnings. Following the
"possible implementation" in the C++ docs, I have replaced the last
remaining aligned_storage_t.

Signed-off-by: Alex Ainscow <[email protected]>

Author: aainscow

src/common/async/waiter.h

@@ -138,8 +138,9 @@ class waiter<> final : public detail::base {
 
 template<typename Ret>
 class waiter<Ret> final : public detail::base {
-  std::aligned_storage_t<sizeof(Ret)> ret;
-
+  struct alignas(Ret) {
+    std::byte data[sizeof(Ret)];
+  } ret;
 public:
   Ret wait() {
     auto l = wait_base();