bump codecov to v6 #205

Job	Run time
build (cpp, macos-latest)	1m 16s
build (js, ubuntu-latest)	1m 20s
build (python, ubuntu-latest)	55s
build (python, macos-latest)	52s
build (cpp, ubuntu-latest)	1m 41s
build (js, macos-latest)	1m 28s
build (rustjswasm, macos-latest)	7m 8s
build (jupyter, macos-latest)	1m 56s
build (rust, ubuntu-latest)	5m 49s
build (cppjswasm, ubuntu-latest)	2m 36s
build (rust, macos-latest)	5m 25s
build (cppjswasm, macos-latest)	2m 28s
build (jupyter, ubuntu-latest)	2m 12s
build (rustjswasm, ubuntu-latest)	7m 35s
	42m 41s

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