gha: bump actions/checkout from 6 to 7 in the github-actions group #887

Job	Run time
sync / sync	45s
sync / commit	32s
check / Lint translations	34s
check / Check for zero-width space characters	5s
check / Build docs (epub)	7m 13s
check / Build docs (latex)	7m 29s
check / Build docs (html)	7m 32s
check / Build docs (pdf)	11m 26s
	35m 36s

Provide feedback