move mt-st from recommended to suggested

mt-st is only used for automatic tape changing via script, and the general audience using tape drives is presumably small.

To reflect this component to be optional and not installed by default, mt-st is moved to be suggested instead of recommended.

In Ubuntu we'd like to move bacula from universe to main, and we'd like to not support mt-st (and need a tape drive to test) for that. I assume Debian could agree in the tape driver not being needed for most users, hence changing the dependency status of mt-st could be done at Debian as well.