Remove support for `node@6` with 12.0

We have been using node >= 8.x for quite some time now. For development we even have switched to node@10.x now.

Should we deprecate support for Node 6, and remove support with 12.0? 12.0 will probably come one month or so before Node 6 EOL.

Node 6 EOL is in April 2019, and we should release 12.0 in March 2019.

We already had breaking builds in the past: https://gitlab.com/gitlab-org/gitlab-ce/issues/50150

Proposal

1. Remove node@6 from our documentation, set the minimum version to 8.
2. Set the minimum version to 8 in the engines property in the package json
3. Remove qa-frontend-node-x job for node@6
4. Add an entry about the removed support to the release blog