Update links to WKT 2 specification

[desruisseaux]

Replace following links to OGC 12-063r5:

http://docs.opengeospatial.org/is/12-063r5/12-063r5.html

by the following links to OGC 18-010r7:

http://docs.opengeospatial.org/is/18-010r7/18-010r7.html

This replacement has already be done in a few places, but more work is needed for the org.opengis.test.wkt package because it contains references to some specific sections of the WKT specification document, which may have changed in the new version.