Update manage_externals in SLIM to latest so that F cases will work

There was an issue in CAM tags that required an update in manage_externals to checkout CAM subcomponents in git rather than partial SVN checkouts. Without this update you can NOT run manage_externals to get a working version of CAM.

The workaround is to update the version of manage_externals in a SLIM checkout, and then you'll be able to checkout CAM. Another way would be to checkout a CESM cesm2_3_alpha17X tag, and then use SLIM in that context.