chore(ci variables): update the ci_variables.json

The GitLab CI variables documentation has changed (seems like a large technical writing review) and we needed to update it.

Docs describing this process: https://gitlab.com/gitlab-org/gitlab-vscode-extension/-/blob/main/docs/ci-variables.md

Also, the documentation format has changed from one table to three tables. This required a small change to the automated script that's parsing this documentation.

What to review?

1. The script change
2. Sanity-check the content of the ci_variables.json file. Its content is automatically generated by parsing the canonical documentation so the check should only focus on looking whether there are any unexpected strings like ------------------ that would indicate incorrect parsing logic.