Update URLs (gmsh.info and gnu.org) to https

This MR is simply changing some of the URLs for gmsh.info and www.gnu.org to use the https protocol instead of the plain text http.

The problem it is trying to solve is that if one visits the official secure website https://gmsh.info/ it tries to load some hard coded image sources as plain text using only http://gmsh.info/, with a browser set to not allow mixed content this will result in no images shown.

The server running gmsh.info is secured with Let's Encrypt certificate meaning it would always* be available over https with a valid certificate.

*Exceptions being normal server and connectivity issues...

The official repository description (at the top of the page https://gitlab.onelab.info/gmsh/gmsh) would also need updating to link to https (I believe it is done in the settings-page of the repository).