Update SRID.csv EPSG:900913

[TimThaler]

from https://epsg.io/900913 WKT

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[airbreather]

We generate SRID.csv from PostGIS (I just pushed a change to update it to the version bundled with PostGIS 3.0.1), and this change isn't there, so I did a bit of research...

900913 is an "unofficial" identifier: it's not in the actual EPSG database, and I doubt that it ever would be given that number, considering that it's just digits that spell out the word "GOOGLE".

Our / PostGIS's version seems to be equivalent to EPSG:3785 (Mercator 1SP on a sphere), which was deprecated in favor of EPSG:3857 (Mercator 1SP on WGS 84).

OpenLayers seems to have been the first group to have actually used the unofficial identifier, and they claim that it's equivalent to EPSG:3857 now, so this is probably OK, but I don't have the time to look into this the rest of the way...