Guide: Add __next40pxDefaultSize to buttons

[t-hamano]

Related to #46741

What?

Change the navigation button of the Guide component to 40px.

Testing Instructions

Test it in one of the following ways:

• Check on StoryBook
• Post Editor > Options Menu > Welcome Guide

Screenshots or screencast

Before	After

[github-actions]

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Co-authored-by: t-hamano <wildworks@git.wordpress.org>
Co-authored-by: mirka <0mirka00@git.wordpress.org>
Co-authored-by: tyxla <tyxla@git.wordpress.org>

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[mirka]

Thank you! 🚀

[tyxla]

LGTM 👍

Thanks for helping with #46741 @t-hamano 🙌