[Feat]: a11y enhancement visual list for sp-field-group

### Code of conduct

- [x] I agree to follow this project's code of conduct.

### Impacted component(s)

sp-field-group

### Description of the requested feature

According to [Adobe storybook](https://opensource.adobe.com/spectrum-web-components/storybook/index.html?path=/docs/field-group--docs), sp-field-group should be using `ul li` list rather than `role="group" for screen reader user.

you can go to this link to check the checkbox items: https://new.express.adobe.com/explore/templates

### Mockups or screenshots

_No response_

### Implementation notes or ideas

_No response_

### Would you like to track this issue in Jira?

- [ ] Yes, please tell me the ticket number!

Start with the sp-field-group implementation and compare its current semantics with the linked Adobe Storybook field-group example. Check the checkbox-list behavior shown in the linked Express page, then verify that the component exposes list semantics correctly for screen-reader users and that existing behavior remains intact.

Written by the indexing model from the issue text.