github: use fedora:40 image for testing #107

Job	Run time
Test with DPDK enabled / test	11m 47s
Test with C++20 modules enabled / test	1m 52s
Test (clang++, C++20, debug) / test	6m 36s
Test (clang++, C++20, dev) / test	4m 24s
Test (clang++, C++20, release) / test	4m 35s
Test (clang++, C++23, debug) / test	6m 42s
Test (clang++, C++23, dev) / test	4m 34s
Test (clang++, C++23, release) / test	4m 41s
Test (g++, C++20, debug) / test	7m 34s
Test (g++, C++20, dev) / test	1m 56s
Test (g++, C++20, release) / test	2m 8s
Test (g++, C++23, debug) / test	7m 20s
Test (g++, C++23, dev) / test	2m 5s
Test (g++, C++23, release) / test	2m 8s
	1h 8m 22s

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