Upgrade to 2.3.0, in order to resurrect freefem++.

Switch to a fork hosted at GitHub:

This GitHub project is designed to provide a common maintained version of
arpackpp. It is derived from the orignial package (ARPACK++ Version 1.2. by
Gomes and Sorensen), which has not been actively maintained for many years.
Several updates have been included in this version (some of them were previously
hosted as patches at http://reuter.mit.edu/software/arpackpatch/ ). This GitHub
repository is designed to collect fixes and updates (e.g. to more recent or
future releases of the involved libraries).

PR: 240670