Remove incorrectly placed GitHub references in GitLab CI page

[MikeMcC399]

• Continuous Integration > GitLab CI incorrectly advises to use a GitLab workflow on GitHub. This makes no sense.

The section referring to applying the GitLab workflow to GitHub is removed. Other inapplicable references to GitHub in a GitLab environment are also removed.

There is no equivalent live replacement repo for GitLab. The original https://gitlab.com/cypress-io/cypress-example-kitchensink/ repo is archived and no longer maintained. Although it is possible that forking the old GitLab repo and applying the workflow to it would still work, there can be no guarantee of this, and it is better to avoid suggesting it, due to uncertain outcomes. (See cypress-io/cypress-example-kitchensink#615 (comment).)

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