README.md: replace C++17 with C++23 #68

Job	Run time
Tests (clang++-18, C++20, dev, )	4m 56s
Tests (clang++-18, C++20, debug, )	6m 45s
Tests (clang++-18, C++20, release, )	5m 19s
Tests (clang++-18, C++23, dev, )	4m 51s
Tests (clang++-18, C++23, debug, --enable-cxx-modules)	2m 51s
Tests (clang++-18, C++23, release, --enable-dpdk)	12m 19s
Tests (gcc-13, C++20, dev, )	4m 35s
Tests (gcc-13, C++20, debug, )	8m 25s
Tests (gcc-13, C++20, release, )	6m 52s
Tests (gcc-13, C++23, dev, )	4m 36s
Tests (gcc-13, C++23, debug, )	7m 58s
Tests (gcc-13, C++23, release, )	5m 45s
	1h 15m 12s

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