chore(deps): update actions/checkout action to v4 #896

Job	Run time
Publish Charm / Generate the Charm Matrix	2s
Run Tests / Bundle Integration	14m 54s
Run Tests / Individual charm's integration tests (controller)	12m 31s
Run Tests / Individual charm's integration tests (ui)	11m 56s
Run Tests / Lint Code (controller)	46s
Run Tests / Lint Code (ui)	21s
Run Tests / Unit Test (controller)	56s
Run Tests / Unit Test (ui)	54s
Run Tests / Check libraries (jupyter-controller) / Check libraries	41s
Run Tests / Check libraries (jupyter-ui) / Check libraries	21s
Run Tests / Check libraries (jupyter-controller) / Lint	9s
Run Tests / Check libraries (jupyter-ui) / Lint	8s
Publish Charm / Publish Charm (./charms/jupyter-ui)	3m 58s
Publish Charm / Publish Charm (./charms/jupyter-controller)	2m 57s
	50m 34s

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