Update Default BCs for regrid.pl?

[mathomp4]

Currently in regrid.pl, the default input and output BCs are Ganymed-4_0:

GMAO_Shared/GEOS_Util/post/regrid.pl

Lines 48 to 49 in 30f555c

	my $former_tag = "Ganymed-4_0"; # default for input restarts
	my $current_tag = "Ganymed-4_0"; # default for output restarts

To me this seems a bit out of date. I think the default output should probably be Icarus-NLv3 as that is the current Land we target in GEOSgcm_App, etc.

As for default input...not sure? Maybe Icarus? To facilitate people regridding their old Icarus restarts to NLv3? (I believe if you ask to regrid from MERRA-2 the Ganymed-4_0 input is selected automatically, so I think we could safely update these. @gmao-jstassi would know better than I would.)