Update actions/checkout action to v6 #3098

Job	Run time
Dependency	4s
Shellcheck	6s
Cache identify	4s
Prepare	8s
Ruff check and force	10s
Check commit	30s
Run pytest using Python 3.14	30s
Run pytest using Python 3.13	34s
Process test coverage	18s
Run mypy	0s
Build and publish Python 🐍 distributions 📦 to TestPyPI	11s
Process test complexity	12s
	2m 47s

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