[Modal] update the close icon to be an icon button

[laurenmrice]

Acceptance criteria

The Modal component in code uses an icon button for "close" with states for a11y reasons. We need to update the Modal component to have this functionality.

• Fix Figma issue for this first

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Usage tab

In general, ensure all images are showing the correct close icon spacing.

• Anatomy: Use the terminology close icon button.

• Alignment: Update the outline around the close icon. Should be 48px.

Style tab

In general, ensure all images are showing the correct close icon spacing.

Structure:

• Image: Update the outline around the close icon. Should be 48px.
• Table: The table should say 20px for the icon, not 16px.
• Remove Plex mono from the structure images, they should be sans.