Fix pipeline schedule CRON for 'gitlab-org/gitlab issues slipping hygiene'

The schedule was running everyday between the 2nd and the 16th, and on all Mondays, but the intended schedule was to only run on Mondays between the 2nd and the 16th, which is impossible to define in CRON syntax.

Instead I'm proposing to post a message only twice a month on 8th and 15th, before the milestone expires (on the 17th).