chore(deps): update actions/checkout action to v4 #901

Job	Run time
Publish Charm / Generate the Charm Matrix	4s
Run Tests / Unit Test (controller)	27s
Run Tests / Unit Test (ui)	57s
Run Tests / Bundle Integration	16m 58s
Run Tests / Individual charm's integration tests (controller)	12m 4s
Run Tests / Lint Code (controller)	19s
Run Tests / Individual charm's integration tests (ui)	13m 14s
Run Tests / Lint Code (ui)	24s
Run Tests / Check libraries (jupyter-controller) / Check libraries	30s
Run Tests / Check libraries (jupyter-controller) / Lint	10s
Run Tests / Check libraries (jupyter-ui) / Check libraries	32s
Run Tests / Check libraries (jupyter-ui) / Lint	12s
Publish Charm / Publish Charm (./charms/jupyter-ui)	3m 38s
Publish Charm / Publish Charm (./charms/jupyter-controller)	3m 4s
	52m 33s

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