Remove 14.04 LTS from installation docs

Installation page lists Ubuntu 14.04 LTS as supported

Ubuntu 14.04 LTS reached 'end of life' April 2019 and is no longer receiving security updates

GitLab stopped building and releasing packages for Ubuntu 14.04 at the same time (version 11.10)

• Issue: gitlab-org/omnibus-gitlab#4234 (closed)
• Merge Request: gitlab-org/omnibus-gitlab!3192 (merged)