Propose to close glibc 2.18 when glibc 2.26 opens.

[Carlos O'Donell]

glibc 2.18 was released in 2013-08-12, 4 years ago.

There isn't much active work on the branch, unlike 2.19, and
up which are still getting backports from interested parties
on both main and vendor branches.

The goal is to reduce the number of open and active branch,
and make it easier for users to know which branches will get
active backports.

Any objections?

-- 
Cheers,
Carlos.