Docs: changing secret variable to just variable in advanced-configuration.md
What does this MR do?
Corrects "Secret Variable" to just "Variable" in the advanced configuration doc.
- Make sure the old link is not removed and has its contents replaced with a link to the new location.
- Make sure internal links pointing to the document in question are not broken.
- Search and replace any links referring to old docs in GitLab Runner's source code