fix: flex justification and sizing on Button

[jaredcwhite]

Description

This is just a couple of small CSS fixes for the Button component as I ran into issues with full-width button update in Core.

How Can This Be Tested/Reviewed?

https://deploy-preview-61--storybook-ui-seeds.netlify.app/ (this should look the same as before, but if you were to edit one of the buttons in browser dev tools to be a specific width like 500px, you'll see that the button is exactly 500px and the text/icon inside is centered)

Checklist:

• [ ] I have added QA notes to the issue with applicable URLs
• [x] I have performed a self-review of my own code
• [x] I have reviewed the changes in a desktop view
• [ ] I have reviewed the changes in a mobile view
• [ ] I have made corresponding changes to the documentation
• [x] My changes generate no new warnings
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] I have exported any new components
• [x] My commit message(s) and PR title are polished in the conventional commit format, and any breaking changes are indicated in the message and are well-described

[netlify[bot]]

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[github-actions[bot]]

:tada: This PR is included in version 1.10.1 :tada:

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot :package::rocket: