Remove support for Docker in runner with CentOS 6

Problem to solve

As discussed at #2609 (comment 58048231), with additional detail at #2176 (comment 26596486), supporting Windows container executors requires a more modern version of the Docker library in the runner. The minimum required version to support Windows also deprecates support for CentOS 6.

CentOS 6 was released on 10 July 2011, stopped receiving full updates on 10 May 2017, and will stop receiving maintenance updates on 30 November 2020.

We would of course retain support for CentOS 7 and above, which is the current version.

Target audience

N/A

Further details

N/A

Proposal

• Upgrade to Docker library which supports Windows. !1148 (merged)
• Update documentation (such as https://about.gitlab.com/install/) will not be done since we are only deprecating docker support #3905 (comment 133952304)
• Deprecate support ahead of time (11.7: gitlab-com/www-gitlab-com!17766 (efc4c13d))

What does success look like, and how can we measure that?

N/A

Links / references