[Accordion] Add new classes key

[natac13]

Breaking change

• The classes.container key was changed to match the convention of the other components.

  -<Collapse classes={{ container: 'collapse' }}>
  +<Collapse classes={{ root: 'collapse' }}>

• I have followed (at least) the PR section of the contributing guide.

closes #21723

[natac13]

@oliviertassinari Should I be changing both the .js file and .tsx file with the changes?

[oliviertassinari]

@natac13 Run

yarn docs:api

[mui-pr-bot]

Details of bundle changes

Generated by 🚫 dangerJS against 0ebbadf

[oliviertassinari]

@natac13 Well done

[eps1lon]

Could somebody elaborate why Collapse was also changed and why the region class was added? This is not apparent from the conversation in this PR or the linked issue.

[oliviertassinari]

@eps1lon The region class was added so all DOM nodes can be targeted in the document. Collapse was changed to fix one inconsistency in the naming.

[eps1lon]

@eps1lon The region class was added so all DOM nodes can be targeted in the document. Collapse was changed to fix one inconsistency in the naming.

Why was this part of this PR?

[oliviertassinari]

Why was this part of this PR?

It was part of this PR to improve the customization story, the topic of the linked issue. It's a small inconsistency we didn't catch sooner (all elements should be customizable), following the boy scouts rule: leaving the forest cleaner than we found it :)