Update actions/checkout action to v6 #5922

Job	Run time
Cache identify	7s
Dependency	5s
Shellcheck	7s
Prepare	7s
Ruff check and force	11s
Check commit	21s
Run pytest using Python 3.13	20s
Run pytest using Python 3.14	15s
Run mypy	17s
Process test coverage	20s
Build and publish Python 🐍 distributions 📦 to TestPyPI	12s
Process test complexity	8s
	2m 30s

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