Ruby: switch default version of Ruby to 2.7.x and devel/rubigen retirement

[Takahiro Kambe <taca%NetBSD.org@localhost>]

Hi,

Before branching of pkgsrc-2021Q2, I'll change default version from
2.6.x to 2.7.x.

* A few packages start to drop 2.6.x support.
* Most of pacakges are now 2.7.x compatible.

And during checking of Ruby packages, I found there is newer version
of devel/rubigen, 1.5.8.  But it was released 9 years ago and it
restricts dependency to activesupport to < 3.2.

So, I will remove devel/rubigen package.

-- 
Takahiro Kambe <taca%NetBSD.org@localhost>