Update actions/checkout action to v6 #3100

Job	Run time
Cache identify	6s
Dependency	3s
Shellcheck	7s
Prepare	1m 22s
Ruff check and force	9s
Check commit	29s
Run pytest using Python 3.14	33s
Run pytest using Python 3.13	1m 33s
Process test coverage	11s
Run mypy	-1s
Process test complexity	11s
Build and publish Python 🐍 distributions 📦 to TestPyPI	10s
	4m 53s

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