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.

まず、bus-factors リポジトリのコンテキストと、この issue で説明されている既存の GMT FTP および Data Server の構成を確認します。tarball を Data Server の releases ディレクトリに移行するために必要な事項として、HTTP/HTTPS アクセス、権限、ミラー、FTP との後方互換性を含めて定義します。移行計画について合意し、その長期的な廃止に関する影響を文書化できた時点を完了とします。

索引モデルが issue の本文から書いたものです。