chore(deps): update actions/checkout action to v4 #835

Job	Run time
Run Tests / Individual charm's integration tests (controller)	15m 39s
Run Tests / Bundle Integration	13m 1s
Run Tests / Individual charm's integration tests (ui)	13m 18s
Publish Charm / Generate the Charm Matrix	6s
Run Tests / Lint Code (controller)	17s
Run Tests / Unit Test (controller)	24s
Run Tests / Lint Code (ui)	20s
Run Tests / Unit Test (ui)	23s
Run Tests / Check libraries (jupyter-controller) / Check libraries	32s
Run Tests / Check libraries (jupyter-controller) / Lint	11s
Run Tests / Check libraries (jupyter-ui) / Check libraries	36s
Run Tests / Check libraries (jupyter-ui) / Lint	17s
Publish Charm / Publish Charm (./charms/jupyter-ui)	4m 52s
Publish Charm / Publish Charm (./charms/jupyter-controller)	4m 34s
	54m 30s

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