Update codecov/codecov-action action to v6 - autoclosed #208

Job	Run time
Build and test (ubuntu-latest, 1.85)	56s
Build no_std for thumbv6m-none-eabi	13s
Build no_std for thumbv7m-none-eabi	18s
Miri (nightly)	1m 9s
Lint	38s
Build and test (ubuntu-latest, stable)	51s
	4m 5s

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