when_all: stop using deprecated std::aligned_union_t #195

Job	Run time
Test with C++20 modules enabled / test	14m 5s
Test with DPDK enabled / test	40m 2s
Test (clang++, C++20, dev) / test	16m 4s
Test (clang++, C++20, debug) / test	20m 54s
Test (clang++, C++20, release) / test	14m 49s
Test (clang++, C++23, debug) / test	40m 12s
Test (clang++, C++23, dev) / test	16m 38s
Test (clang++, C++23, release) / test	15m 34s
Test (g++, C++20, debug) / test	35m 47s
Test (g++, C++20, dev) / test	13m 8s
Test (g++, C++20, release) / test	16m 40s
Test (g++, C++23, debug) / test	26m 31s
Test (g++, C++23, dev) / test	13m 12s
Test (g++, C++23, release) / test	17m 0s
	5h 0m 36s

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