Foundation: Update Accordion template element naming

[bheston]

Pull Request

📖 Description

As part of the next major release of fast-foundation, we wanted to do a complete review of the component templates and align the structure and naming as much as possible.

This PR is split from #6286 and applies to Accordion and Accordion Item.

Before on the left, update on the right. "C" = class, "P" = part, "R" = role, "S" = slot. 💔 = potentially style-breaking.

Accordion

• Remove unused <template> wrapper

Accordion item

• 💔 Rename “icon” to “expand-collapse-icon”
• 💔 Rename "region" class and part to "panel"

👩‍💻 Reviewer Notes

Review the Storybook site, or the illustrations of the components before and after, compared to the changes in code.

📑 Test Plan

Tested via comparison against proposal and in Storybook.

✅ Checklist

General

• I have included a change request file using $ yarn change
• I have added tests for my changes.
• I have tested my changes.
• I have updated the project documentation to reflect my changes.
• I have read the CONTRIBUTING documentation and followed the standards for this project.

Component-specific

• I have added a new component
• I have modified an existing component
• I have updated the definition file
• I have updated the configuration file