Move `ci_variable_list` directory to `ci/ci_variable_list`

What does this MR do and why?

For #381522 (closed)

Moves ci_variable_list files under the ci directory, following the change in the CODEOWNERS file for Verify frontend code to be in the same directory.

Screenshots or screen recordings

How to set up and validate locally

• Pipeline should pass
• User should be able to add/update/delete variables from the Admin/Group/Project CI/CD settings (Settings > CI/CD > Variables)

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.