fix(Accordion): update Accordion styles to match spec

[tw15egan]

Closes #12141
Closes #12223

Fixes a few style issues with Accordion

Changelog

Changed

• lg size Accordion is now center aligned
• Background hover color is now $background-hover, not $layer-hover

Testing / Reviewing

• Go to the Playground story and set the size to lg, ensure the text and chevron are aligned
• Hover over the AccordionItem and ensure the token is $background-hover on the ::before element

cc @shixiedesign

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[shixiedesign]

Both issues are fixed! Thank you @tw15egan 🎉 🎉 🎉

[aagonzales]

The only thing about using background-hover is that its transparent and the border dividers now show up with the hover.

Where as before they were blended in/hidden.

[tw15egan]

@aagonzales, do we just need to update the spec on the website re: background-hover? lg fix can remain in the PR, but if its just a spec change I can PR the website to update the token value

[aagonzales]

Yeah after looking at it a little more I think we should keep it $layer-hover in code (its using layer-hover in both figma and sketch as well) and just update the website to reflect that. @tw15egan