chore(ci variables): update the ci_variables.json

This MR is the result of running the script to update the CI variable definition. There is no manual editing in this commit. The content of ci_variables.json is scraped from the official GitLab documentation.

Documentation: https://gitlab.com/gitlab-org/gitlab-vscode-extension/blob/5adb82a3cc7f4d8b65b2e4886c26a1c6d09a7ae3/docs/ci-variables.md#L10

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I also added the above text to our documentation so the next time we have to create this MR, it's a bit easier.