fix: collapse AccordionContent child margins into panel padding

[Devil1716]

Summary

• Zero out top margin on the first child and bottom margin on the last child of Accordion__panel
• Prevents child <p> (and similar) margins from stacking on top of the panel's 16px padding

Closes #2482

Test plan

• Open Accordion with <p> children inside AccordionContent
• Confirm spacing from panel edge to text is ~16px (panel padding only), not padding + paragraph margin

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[aws-amplify-us-east-1]

This pull request is automatically being deployed by Amplify Hosting (learn more).

Access this pull request here: https://pr-2483.d15792l1n26ww3.amplifyapp.com