Provide `no-redundant-roles` exception for `rowgroup`

[khiga8]

Relates to: https://github.com/github/accessibility/issues/5304

Follow-up to: https://github.com/github/accessibility/discussions/4921

What

This PR adds an exception so that the no-redundant-roles rule does not flag redundant use of rowgroup for:

• <thead role="rowgroup">
• <tbody role="rowgroup">

Why

There have been 3 reported instances of false positives raised by this rule. All of these relate to redundant usage of role="rowgroup". Redundant usage of role="rowgroup" is necessary in some cases to ensure that table semantics are not dropped by screen readers.

The reported instances are:

• a screen reader/browser bug where table semantics were being dropped when display: grid or display: contents is used. Example - example in primer/react
• a screen reader/browser bug where table semantics were being dropped when table header is missing. (Related: https://github.com/github/github/pull/261899#discussion_r1121141272)

It seems appropriate to not flag redundant use of role="rowgroup" for now. We can always revisit this in the future.

Why don't we turn this rule off completely?

All the reported false positives are for role="rowgroup" which we can except, so it does not seem worth turning the rule off completely.

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