Update CI docs

[na--]

A user commented on the approach we suggest for using k6 in GitLab CI - our examples (https://gitlab.com/loadimpact/k6-gitlab-example, https://loadimpact.com/integrations/performance-testing-gitlab/#configuration) use wget to download a very old (v0.21.1) k6 version instead of the official docker image.

cc @ppcano, @robingustafsson, @markjmeier

[na--]

A cursory look at the other CI integration pages reveals other obsolete details:

• the Jenkins page and example suggest users to set K6CLOUD_TOKEN, which is obsolete. It will still mostly work, but users will see a warning that they should use K6_CLOUD_TOKEN instead.
• The CircleCI page advocates for k6 v0.15.0 :astonished: That one was released on May 19 2017 :man_facepalming: In any case, I'm pretty certain that CircleCI would be better served by a containerized k6 as well.
• The TeamCity page also uses K6CLOUD_TOKEN
• All of these things are screenshots, when they would be better off as copy-paste-able (and easily editable) code snippets.

[na--]

Not sure if any of this is still relevant after the new docs, K6CLOUD_TOKEN still seems to be present in some files: https://github.com/search?q=org%3Aloadimpact+K6CLOUD_TOKEN&type=Code

[simskij]

Yeah, this definitely needs to be updated. Good catch!

[ppcano]

The CI content was updated. Closing the issue.