Upgrade dataset-check.yml to check new realese of SRTM15

The [dataset-check.yml](https://github.com/GenericMappingTools/gmtserver-admin/blob/master/.github/workflows/dataset-check.yml) workflow checks for SRTM15 new releases.

If I understand it correctly, it looks for the file SRTM15_V2.4.nc on this server (https://topex.ucsd.edu/pub/srtm15_plus/). If it founds it, them it is ok. If not, them looks for a newer version.

The issue is that the v2.4 still is on the server altough two new versions have been added (v2.5 and v2.5.5).


So, this workflow should be modified. I am not sure how to do it but I could try. Also, I am not sure what is a better way to check for newer versions. Maybe to check the date?

BTW, I think that we could make a similar workflow for the gravity grids (FAA and VGG). Is it ok to add it to the same workflow? Or is it better to make a new one?