Merge the GMT FTP server into Data Server?

The GMT FTP site (ftp://ftp.soest.hawaii.edu/gmt) is the site for distributing GMT tarballs. Before we migrated to GitHub, it's the only official site for downloading GMT tarballs.

As mentioned in the bus-factors repository, it has a bus factor = 1, as Paul is the only person who can manage files in the FTP server.

Considering that "FTP is almost dead" (https://filecamp.com/blog/top-5-reasons-ftp-dead/, https://www.ghacks.net/2019/08/16/google-chrome-82-wont-support-ftp-anymore/), should we migrate the GMT FTP server to the Data Server? For example, put all tarballs in the `releases` subdirectory?

**Pros**:

1. the Data server uses HTTP or HTTPS, not FTP.
2. @PaulWessel @joa-quim @leouieda @seisman all can manage the files in the data server, so the bus factor is 4.
3. The data server already have many mirrors

We can still keep the FTP server for backward compatibility, but we may retire it after 5-10 years.

Comienza revisando el contexto del repositorio bus-factors y las configuraciones existentes de GMT FTP y Data Server descritas en este issue. Define qué requeriría mover los tarballs a un directorio de releases en un Data Server, incluido el acceso mediante HTTP/HTTPS, los permisos, los mirrors y la compatibilidad retroactiva con FTP. Se considerará terminado cuando el plan de migración esté acordado y sus implicaciones para la retirada a largo plazo estén documentadas.

Escrito por el modelo de indexación a partir del texto del issue.