[PROJ] Motion: adopt RFC 5

[Even Rouault]

Hi,

RFC 5 implementation has now been merged into PROJ master, and in the 
https://github.com/OSGeo/PROJ-data repository (was known transiently as proj-
datumgrid-geotiff), which is now the new home for grids & similar datasets.

https://github.com/OSGeo/PROJ-data/tree/master/grid_tools contains the tools & 
documentation to create Geodetic TIFF grids (GTG)

And https://github.com/OSGeo/PROJ-data/blob/master/CONTRIBUTING.md contains 
the workflow to follow for adding new data.

For now, https://cdn.proj.org/ contains both the initial grid names (like 
egm96_15.tif), and the new ones following RFC 5 naming scheme 
(us_nga_egm96_15.tif)
I'll remove next week the old files, so for the initial adopters of RFC 4, 
please update to latest master now, so proj.db points to the new names.

Even

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