Revise hgroup allowances

[scottaohara]

Once implementations land for https://github.com/w3c/html-aam/pull/398, hgroup will map to the group role.

Updates for this spec:

• [ ] the implicit mapping will need to be changed from generic to group
• [ ] the naming allowance will need to be updated
• [ ] checker issues will need to be filed per the above changes

[scottaohara]

resolved via https://github.com/w3c/html-aria/pull/463