feat(Modal): update Modal styles to phase 3 spec

[tw15egan]

Closes #15955

Updates Modal and ComposedModal to the phase 3 spec

Changelog

Changed

• Renders the sm Slug by default inside Modal
• Adjusted positioning based on new size

Removed

• Removed hasScrollingContent prop from Slug story as this is now handled internally (fix(Modal): add tabindex to content body when content is scrollable, storybook enhancements  #15515)

Testing / Reviewing

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[tay1orjones]

@aagonzales this one's ready for you to review

[aagonzales]

The modal itself looks good but I noticed there were some bugs in the slug coming through. Not sure if that's related to this PR or not so I'll go ahead and approve.

[tw15egan]

@aagonzales, I think the button underneath may affect the filter blur on the action bar 😬