gha: bump actions/checkout from 5 to 6 in the github-actions group (#… #306

Job	Run time
sync / sync	35s
check / Lint translations	54s
check / Check for zero-width space characters	42s
check / Build docs (latex)	7m 34s
check / Build docs (html)	7m 36s
check / Build docs (epub)	7m 27s
check / Build docs (pdf)	11m 32s
	36m 20s

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