[Accordion] Fix invalid HTML inside heading

[ZeeshanTamboli]

Fixes #44153

This PR resolves a regression introduced in #42914 by making the following changes to ensure valid HTML structure for the Accordion component:

• Changed the Accordion summary element to button.
• Updated the Accordion summary content and icon wrapper elements to span.

Additionally, typography has been removed from the demos since the p element rendered by Typography is invalid inside a span. Typography styles are now applied directly to the summary button using theme.typography.body1, making the extra Typography component unnecessary.

Breaking Change

• The structure of the Accordion summary has been updated:
  • The root element is now a button.
  • Summary content and the icon wrapper are rendered as span.
• Developers using the previous div element for styling in the AccordionSummary should update their styling. Additionally, those using Typography, which defaults to rendering a p tag, should replace it with a span or remove it for valid HTML, as shown in the updated demos.

These changes are critical to fix the regression but introduce a breaking change. IMO, this should be released in a minor version (v6.x) and documented in the migration guide. I know this is not ideal but the fix is also important.

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If approved, I’ll include a note in the migration guide detailing these changes.

[mui-bot]

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