gha: bump actions/checkout from 5 to 6 in the github-actions group #304

Job	Run time
sync / sync	41s
check / Lint translations	45s
check / Build docs (html)	6m 58s
check / Build docs (latex)	7m 41s
check / Check for zero-width space characters	7s
check / Build docs (epub)	7m 14s
check / Build docs (pdf)	10m 26s
	33m 52s

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