fix(material/stepper): updates vertical-stepper aria roles

[essjay05]

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Updates Angular Component's vertical stepper to use more generic aria roles since having the default tablist/tab/tabpanel applied to the vertical stepper violates WCAG rules of having tabpanel as a nested child within tablist. The new roles of group and region satisfy aria requirements while maintaining the current html structure of the vertical stepper.

Fixes b/361783174