fix(fieldset): invalid fieldset HTML fix

[nuria1110]

BREAKING CHANGE: The inline prop has been removed from the Fieldset component.

Proposed behaviour

• Legend element is no longer nested in two div containers and is now a direct child of the fieldset element.
• inline prop has been removed.

Current behaviour

• Legend element in the Fieldset component is nested in two divs, which is invalid HTML.

Checklist

• [x] Commits follow our style guide
• [ ] Related issues linked in commit messages if required
• [ ] Screenshots are included in the PR if useful
• [ ] All themes are supported if required
• [x] Unit tests added or updated if required
• [x] Playwright automation tests added or updated if required
• [ ] Storybook added or updated if required
• [ ] Translations added or updated (including creating or amending translation keys table in storybook) if required
• [ ] Typescript d.ts file added or updated if required
• [ ] Related docs have been updated if required

QA

• [ ] Tested in provided StackBlitz sandbox/Storybook
• [ ] Add new Playwright test coverage if required
• [ ] Carbon implementation matches Design System/designs
• [ ] UI Tests GitHub check reviewed if required

Additional context

Testing instructions