fix(button): correct the width and height of icons inside buttons

[harpalsingh]

Proposed behaviour

Correct the icon sizes when used inside a button to 20px width and height.

Current behaviour

Icons inside button are currently using "space" as 16px width and height.

Visual to show changes

Checklist

• Commits follow our style guide
• Related issues linked in commit messages if required
• Screenshots are included in the PR if useful
• All themes are supported if required
• Unit tests added or updated if required
• Cypress automation tests added or updated if required
• Playwright automation tests added or updated if required
• Storybook added or updated if required
• Translations added or updated (including creating or amending translation keys table in storybook) if required
• Typescript d.ts file added or updated if required
• Related docs have been updated if required

QA

• Tested in CodeSandbox/storybook
• Add new Cypress test coverage if required
• Carbon implementation matches Design System/designs
• UI Tests GitHub check reviewed if required

Additional context

n/a

Testing instructions

View Button story and view button with an icon, inspect to view size render.

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[carbonci]

🎉 This PR is included in version 123.4.3 🎉

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