Signed-off-by: NRémy Coutable <remy@rymai.me>

Given that the 9.0 release of GitLab will mark the end of the support
for Ruby 2.1, we can stop testing against Ruby 2.1.

Fixes #27251

[ci skip]

Ruby 2.3:
```
URI.parse('http://foo/bar?baz quux')
=> #<URI::HTTP http://foo/bar?baz%20quux>
```

Ruby 2.1:
```
URI.parse('http://foo/bar?baz quux')
URI::InvalidURIError: bad URI(is not URI?): http://foo/bar?baz quux
```

We usually use it for security fixes that don't need to be ported to EE
and the task seems to hang when run against the security branch.
Signed-off-by: NRémy Coutable <remy@rymai.me>

This reverts commit 23fc7875.

AKA:
- move all conversations to #core
- beating will continue until morale improves
- Learn by pain
- Pain driven development