[PROJ] Motion: promote PROJ 6.3.1RC1 to final release

[Even Rouault]

PSC Members,

Issue https://github.com/OSGeo/PROJ/issues/1925 has been raised regarding a 
change of behavior related to Dutch grids. The EPSG v9.8.6 upgrade in 6.3.1 
now supersedes the 2008 Dutch (non free) grids by the 2018 ones (CC BY 4.0). 
That change impact results when using the automatic pipeline computations like 
cs2cs, hopefully for the best. We didn't ship the 2008 grids in
proj-datumgrid-europe because of their license, and we don't yet ship the 2018 
ones in proj-datumgrid-europe (the update has been done only in the new PROJ-
data geotiff based directory), so users have to manually install them directly 
from the data producer or the Debian proj-rdnap package, which doesn't change 
a lot the situation since they already had to do that for the 2008 grids.

So all in all,

Motion: promote PROJ 6.3.1RC1 to final release

Starting with my +1

Even

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