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.