Update Externals.cfg to use uppercase repository names

[billsacks]

@fischer-ncar - while not necessary, it will keep things more consistent if Externals.cfg is updated at some point to use the new, uppercase repository names in the repo URLs.

The relevant changes I made yesterday are:

• ctsm -> CTSM
• cism-wrapper -> CISM-wrapper
• rtm -> RTM
• mosart -> MOSART