THIRTY_DAY_MONTHS calendar is not CF compliant

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Describe the bug
Using the THIRTY_DAY_MONTHS calendar leads to a CF-non-compliant calendar attribute in the netCDF file, namely time:calendar = "THIRTY_DAY_MONTHS" ;. For this calendar the CF-compliant attribute is time:calendar = "360_day" ;. The CF calendar attribute is documented at https://cfconventions.org/Data/cf-conventions/cf-conventions-1.8/cf-conventions.html#calendar

To Reproduce
Use THIRTY_DAY_MONTHS from time_manager_mod with

use time_manager_mod, only : THIRTY_DAY_MONTHS

Run a model with the THIRTY_DAY_MONTHS calendar selected and examine the output. If you don't have examples of the above then

git clone --recursive https://github.com/ocean-eddy-cpt/NeverWorld2
cd NeverWorld2
(. scripts/gaea-gnu-env ; make TEMPLATE=src/mkmf/templates/ncrc-gnu.mk -j )
cd config_0p25/
srun -n 800 ../build/MOM6  # On compute node
ncdump -h snapshots_00000005.nc

Expected behavior
In any output file from diag_manager with a time-dependent variable we expect to get (CDL):

	double time(time) ;
		time:long_name = "time" ;
		time:units = "days since 0001-01-01 00:00:00" ;
		time:cartesian_axis = "T" ;
		time:calendar_type = "360_day" ;
		time:calendar = "360_day" ;
		time:bounds = "time_bnds" ;

but actually get

	double time(time) ;
		time:long_name = "time" ;
		time:units = "days since 0001-01-01 00:00:00" ;
		time:cartesian_axis = "T" ;
		time:calendar_type = "THIRTY_DAY_MONTHS" ;
		time:calendar = "THIRTY_DAY_MONTHS" ;
		time:bounds = "time_bnds" ;

System Environment
Describe the system environment, include:

• OS: any
• Compiler(s): any
• MPI type, and version (e.g. MPICH, Cray MPI, openMPI): any
• netCDF Version: For both C and Fortran: any
• Configure options: any

Additional context
This has been raised in MOM6 issue NOAA-GFDL/MOM6/issues/1203 in the context of reading output with xarray. OUr non-compliance with CF has been lond-standing and needs to be addressed.