README.md: replace CircleCI badge with GitHub badge #90

Job	Run time
Tests (clang++-18, C++20, dev, )	8m 48s
Tests (clang++-18, C++20, debug, )	11m 12s
Tests (clang++-18, C++20, release, )	9m 13s
Tests (clang++-18, C++23, dev, )	9m 32s
Tests (clang++-18, C++23, debug, --enable-cxx-modules)	5m 35s
Tests (clang++-18, C++23, release, --enable-dpdk)	19m 35s
Tests (gcc-13, C++20, dev, )	7m 51s
Tests (gcc-13, C++20, debug, )	12m 24s
Tests (gcc-13, C++20, release, )	12m 15s
Tests (gcc-13, C++23, dev, )	8m 24s
Tests (gcc-13, C++23, debug, )	11m 25s
Tests (gcc-13, C++23, release, )	9m 36s
	2h 5m 50s

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