Fix focus style of buttons in page menu

[krysal]

Following up #1191 that left out the items in the pages menu, as @obulat noticed.

Description

This PR makes the VItem component use always the variant plain of the VButton to keep focus-visible styles. I don't know in which cases the "plain-dangerous" version would be used, please point it out or describe it if you find it.

Testing Instructions

Navigate the pages menu in a version of Safari < 15.4 with the keyboard, you should see the focus ring in each page entry.

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[obulat]

I first looked at how it worked, and then opened the code. Didn't realize that the code change was so small :) Works very well!

[sarayourfriend]

plain-dangerous was meant as a way to have a button with all the accessibility stuff bolted on but without any styles whatsoever, including focus styles.

This was useful for VItem because the designs for some breakpoints have the focus ring inside the button with the grey outline of the active button appearing around it, generally.

However, we don't use that version of the item anywhere in the app. And this actually fixes the focus outline for VItem in storybook on Firefox too!

[sarayourfriend]

LGTM!

[krysal]

Thanks for explaining the reason behind the variant, @sarayourfriend! I guess we can revise it when the need arises.