README.md: replace CircleCI badge with GitHub badge #91

Job	Run time
Tests (clang++-18, C++20, dev, )	6m 15s
Tests (clang++-18, C++20, debug, )	8m 39s
Tests (clang++-18, C++20, release, )	8m 32s
Tests (clang++-18, C++23, dev, )	12m 54s
Tests (clang++-18, C++23, debug, --enable-cxx-modules)	5m 59s
Tests (clang++-18, C++23, release, --enable-dpdk)	19m 34s
Tests (gcc-13, C++20, dev, )	11m 33s
Tests (gcc-13, C++20, debug, )	12m 28s
Tests (gcc-13, C++20, release, )	7m 26s
Tests (gcc-13, C++23, dev, )	11m 32s
Tests (gcc-13, C++23, debug, )	8m 35s
Tests (gcc-13, C++23, release, )	13m 25s
	2h 6m 52s

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