CTSM to transition from manage_externals to git submodules

[glemieux]

There is a draft pull request in the works for CTSM (ESCOMP/CTSM#2303) that will remove the manage_externals mechanism of checking out the externals for a given ctsm tag and replace it with the use of the git submodule command. This is also how E3SM manages externals and is being changed to align with that method. There is currently no official date for when this is expected to be integrated, but I wanted to give folks a heads up.