Update codecov/codecov-action action to v6 #2392

Job	Run time
flake8	14s
uv-check	7s
ruff-lint	8s
pymarkdown	9s
pre-commit / autofix-pre-commit	2m 7s
mypy (tests)	25s
mypy (package)	30s
pytest	17s
build-and-publish	48s
release	0s
	4m 45s

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