fix(expandable-section): use link button, update styles

[mcoker]

fixes #6998

Visual regression report - expandable-section.pdf

Just a draft to see if it's something we want to get in this sprint. Cleans up some styles, too.

The react changes would be:

• Make __toggle a <div> instead of a custom button
• Use a link <Button> in __toggle and update old __toggle logic to apply to the <Button> instead

[patternfly-build]

Preview: https://patternfly-pr-7000.surge.sh

A11y report: https://patternfly-pr-7000-a11y.surge.sh

[lboehling]

very nice!! ✨

[srambach]

Love removing extra styling :)

[mcoker]

@tlabaj before we commit this in core, are you OK with this change and the react follow-up needed? TL;DR

• I reviewed expandable section with Lucia and we noted some visual discrepancies with the original design intent, so I made this PR as a spike to see what the lift is to align with the design intent. I'm good with it going in on the core side, it was a simple update.
• I think the react update is to
  • Remove expandable section's custom toggle and replace it with a link <Button>
  • Move the logic that was on the custom toggle to the <Button> (here's the core component logic on button)
  • Add a <div class="pf-v6-c-expandable-section__toggle> wrapper around the button.

[patternfly-build]

🎉 This PR is included in version 6.0.0-alpha.223 🎉

The release is available on:

• GitHub release
• npm package (@alpha dist-tag)

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