Refactor MR widget merge checkboxes to more flex

What does this MR do?

In !55905 (merged) we introduced some refs to gl-min-h-7 which coupled the elements to a specific height. This MR removes that coupling by making the parents a bit more flex-y.

Screenshots (strongly suggested)

With checkboxes:

Before	After

Responsive:

Before	After

With message:

Before	After

Does this MR meet the acceptance criteria?

Conformity

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• Documentation (if required)
• Code review guidelines
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• Style guides
• Database guides
• Separation of EE specific content