Fix a subtle issues with squashing the docs-ci history (#7075)

Fix a subtle issues with squashing the docs-ci history (#7075)

The docs-cleanup job squashes all commits from the docs-ci repository that are older than one month. The current solution to retrieve the newest commit older than this one month erroneously relied on the commit date. As the script cherry-picks all newer commits, it should rather use the author date, though. Unfortunately, rev-list does not support filtering by author date, hence we use awk now...