Guide: Update finish button to use the new default size

[tyxla]

What?

This PR updates the "Finish" button of the Guide component to use the new 40px default size.

Why?

Discovered as I was auditing button sizes as part of reviewing #65018.

How?

We're adding the __next40pxDefaultSize prop to the "Finish" button. The rest of the buttons already used that size, so we're just making all buttons consistent now.

Testing Instructions

• Open Storybook: /?path=/docs/components-guide--docs
• Open the guide.
• Go to the last step.
• Verify the "Finish" button is the same size as the "Previous" button (40px).

Testing Instructions for Keyboard

Same

Screenshots or screencast

Before	After

[github-actions]

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Co-authored-by: tyxla <tyxla@git.wordpress.org>
Co-authored-by: ciampo <mciampini@git.wordpress.org>

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[ciampo]

🚀

[github-actions]

There was a conflict while trying to cherry-pick the commit to the wp/6.7 branch. Please resolve the conflict manually and create a PR to the wp/6.7 branch.

PRs to wp/6.7 are similar to PRs to trunk, but you should base your PR on the wp/6.7 branch instead of trunk.

# Checkout the wp/6.7 branch instead of trunk.
git checkout wp/6.7
# Create a new branch for your PR.
git checkout -b my-branch
# Cherry-pick the commit.
git cherry-pick b1f07db58b9b9dcf5068a7e88e63dbf845cef91b
# Check which files have conflicts.
git status
# Resolve the conflict...
# Add the resolved files to the staging area.
git status
git add .
git cherry-pick --continue
# Push the branch to the repository
git push origin my-branch
# Create a PR and set the base to the wp/6.7 branch.
# See https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/proposing-changes-to-your-work-with-pull-requests/changing-the-base-branch-of-a-pull-request.

[tyxla]

@noisysocks if I understand correctly, this has already been backported, but let me know if I can help with anything.