Fix prominent create board button alignment

What does this MR do and why?

When a scrollbar appears in the search bar on a board (due to the window width being too small, the experimental prominent create board button alignment is incorrect. This change makes the button align with the top of the container, rather than the center to keep the alignment consistent. See screenshots for example.

MR acceptance checklist

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Screenshots or screen recordings

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Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

I was able to enable the feature flag for the experimental prominent create board button, but the button wouldn't appear for me.

You can see the problem this board and then use the browser devtools to edit the class changed in this MR.