[PROJ] Motion: approve RFC3 update, and bump to C++17 for PROJ 9.6

[Even Rouault]

Hi,

Motion adopt https://github.com/OSGeo/PROJ/pull/4369 changes to RFC-3 to 
relax it to allow updating programming language standard in minor 
versions, and update build requirements for PROJ 9.6 to be C++17.

+1

Even

-- 
http://www.spatialys.com
My software is free, but my time generally not.
Butcher of all kinds of standards, open or closed formats. At the end, this is just about bytes.