Fix: Improve VSA edit modal responsiveness

What does this MR do and why?

Fixes #349008 (closed)

• Switches the flex direction to column for small screens
• Re-arranges the template to take advantage of the flex direction change
• Adds gl-sm-mt-6 utility class which will be moved to gitlab-ui in a follow-up MR

Screenshots

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How to set up and validate locally

#349008 (closed)

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