Enhance CI/CD variable drawer UI with popovers for help content

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What does this MR do and why?

Enhance CI/CD variable drawer UI in the group/project settings with popovers for help content.

Screenshots or screen recordings

Before	After
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How to set up and validate locally

1. Navigate to the group/project CI/CD settings.
2. Expand Variables section.
3. Click an Add variable button.
4. In the opened drawer validate the help icon behaviour near Environments and Visibility labels.

Related to #499541 (closed)