Switch default version of Ruby from 2.6 to 2.7.

For detailed list of changes, please check the Ruby website:
https://www.ruby-lang.org/en/news/2019/12/25/ruby-2-7-0-released/
https://www.ruby-lang.org/en/news/2020/03/31/ruby-2-7-1-released/
https://www.ruby-lang.org/en/news/2020/10/02/ruby-2-7-2-released/

How to upgrade is described in UPDATING entry 20201128.

PR: 244787
Submitted by: meta@
Exp-Run: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=244787
Thanks to: antoine, meta