Database: register geoid file for UK added in OSGeo/PROJ-data#25

[and-marsh]

Register geoid file for UK added in OSGeo/PROJ-data#25

There are already existing transformations ETRS89 to ODN height (2) with code EPSG:7711
that use this grid OSTN15_OSGM15_GB.txt. So that's enough to add only grid_alternatives entry

[hernando]

Thanks for this PR! This was in my TODO list since I opened #2173. Could you please add:

INSERT INTO "geoid_model" SELECT 'OSGM15', auth_name, code FROM grid_transformation WHERE auth_name = 'EPSG' AND grid_name LIKE '%OSGM15%' AND deprecated = 0;

to the section for geoid models in customizations.sql as well?

[and-marsh]

@hernando, thank you!
Of course I will add your suggestion, but first i need to resolve all issues related to OSGeo/PROJ-data#25