Update FSNT variable for obs data set

The previous variable `toa_sw_all_mon` was incorrect for representing FSNT. The obs file `CERES_EBAF_Ed4.1_2001-2020.nc` now has variable fsnt which is solar_mon - toa_sw_all_mon and:

toa_sw_all_mon -> "toa_outgoing_shortwave_flux" ;
solar_mon - >"toa_incoming_shortwave_flux" ;

lib/adf_variable_defaults.yaml

@@ -895,7 +895,7 @@ FSNT:
       label : "Wm$^{-2}$"
   obs_file: "CERES_EBAF_Ed4.1_2001-2020.nc"
   obs_name: "CERES_EBAF_Ed4.1"
-  obs_var_name: "toa_sw_all_mon"
+  obs_var_name: "fsnt"
   category: "TOA energy flux"
 
 FSNTC: