Update codecov/codecov-action action to v6 #2396

Job	Run time
ruff-lint	11s
uv-check	7s
pymarkdown	10s
flake8	9s
pre-commit / autofix-pre-commit	2m 2s
pytest	21s
mypy (package)	29s
mypy (tests)	20s
build-and-publish	46s
release	0s
	4m 35s

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