Ocean_BGC checkout needs update

[laurenchilutti]

Describe the bug
ocean_BGC repository's master branch was renamed "main". We need to update our checkout script.

SHiELD_build/CHECKOUT_mom6.csh

Line 12 in 3311038

set git_output=`git clone -q --recursive -b master https://github.com/NOAA-GFDL/ocean_BGC.git >& /dev/stdout`

To Reproduce
Try to checkout shiemom ./CHECKOUT_code shiemom and it will fail

Expected behavior
Code should check out

System Environment
Tested on Gaea

Additional context
N/A