Make delete-days delete only files with mailcow-* in backup location (#3609)

Co-authored-by: Florian Lindner <florian.lindner@ipvs.uni-stuttgart.de>

helper-scripts/backup_and_restore.sh

@@ -130,7 +130,7 @@ function backup() {
     --delete-days)
       shift
       if [[ "${1}" =~ ^[0-9]+$ ]]; then
-        find ${BACKUP_LOCATION}/* -maxdepth 0 -mmin +$((${1}*60*24)) -exec rm -rvf {} \;
+        find ${BACKUP_LOCATION}/mailcow-* -maxdepth 0 -mmin +$((${1}*60*24)) -exec rm -rvf {} \;
       else
         echo "Parameter of --delete-days is not a number."
       fi