fix(RadioButton): remove aria-readonly from fieldset

[tw15egan]

Closes #13665

Removes aria-readonly from the underlying fieldset in RadioButtonGroup

Changelog

Removed

• Removed aria-readonly from fieldset

Testing / Reviewing

Ensure no a11y violations appear when readOnly is set to true in Radio Button story.

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[francinelucca]

Confirmed can no linger see violation

[carbon-bot]

Hey there! v11.31.0 was just released that references this issue/PR.