README.md: replace CircleCI badge with GitHub badge #58

Job	Run time
Tests (clang++-18, C++20, dev, )	11m 2s
Tests (clang++-18, C++20, debug, )	12m 51s
Tests (clang++-18, C++20, release, )	8m 28s
Tests (clang++-18, C++23, dev, )	8m 44s
Tests (clang++-18, C++23, debug, --enable-cxx-modules)	6m 20s
Tests (clang++-18, C++23, release, --enable-dpdk)	5m 0s
Tests (gcc-13, C++20, dev, )	9m 29s
Tests (gcc-13, C++20, debug, )	12m 25s
Tests (gcc-13, C++20, release, )	9m 11s
Tests (gcc-13, C++23, dev, )	13m 8s
Tests (gcc-13, C++23, debug, )	11m 16s
Tests (gcc-13, C++23, release, )	9m 11s
	1h 57m 5s

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