github: use fedora:40 image for testing #106

Job	Run time
Test with DPDK enabled / test	11m 7s
Test (clang++, C++20, dev) / test	13m 58s
Test (clang++, C++20, debug) / test	6m 37s
Test with C++20 modules enabled / test	2m 41s
Test (clang++, C++20, release) / test	4m 51s
Test (clang++, C++23, dev) / test	14m 1s
Test (clang++, C++23, release) / test	15m 54s
Test (clang++, C++23, debug) / test	12m 39s
Test (g++, C++20, debug) / test	7m 33s
Test (g++, C++20, dev) / test	5m 21s
Test (g++, C++20, release) / test	1m 51s
Test (g++, C++23, debug) / test	7m 3s
Test (g++, C++23, dev) / test	5m 37s
Test (g++, C++23, release) / test	1m 57s
	1h 51m 10s

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