Remove 2.5 (EOL)

https://www.ruby-lang.org/en/downloads/branches/

https://www.ruby-lang.org/en/news/2021/04/05/ruby-2-5-9-released/

> After this release, Ruby 2.5 reaches EOL. In other words, this is the last release of Ruby 2.5 series. We will not release Ruby 2.5.10 even if a security vulnerability is found. We recommend all Ruby 2.5 users to upgrade to Ruby 3.0, 2.7 or 2.6 immediately.

Author: tianon