README.md: replace C++17 with C++23 #64

Job	Run time
Tests (clang++-18, C++20, dev, )	18m 4s
Tests (clang++-18, C++20, debug, )	16m 56s
Tests (clang++-18, C++20, release, )	17m 36s
Tests (clang++-18, C++23, dev, )	18m 17s
Tests (clang++-18, C++23, debug, --enable-cxx-modules)	13m 46s
Tests (clang++-18, C++23, release, --enable-dpdk)	24m 53s
Tests (gcc-13, C++20, dev, )	4m 48s
Tests (gcc-13, C++20, debug, )	7m 43s
Tests (gcc-13, C++20, release, )	6m 17s
Tests (gcc-13, C++23, dev, )	5m 40s
Tests (gcc-13, C++23, debug, )	8m 19s
Tests (gcc-13, C++23, release, )	6m 7s
	2h 28m 26s

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