Restricted allowed heading for popover slot

[khiga8]

Closes #491

What

This PR restricts the allowed tags for popover component heading slot to heading tags. This is the last of components and slots needing tag restrictions.

Notes

Usually, I don't think it's great to have a default for heading since the appropriate heading level tends to be context-dependent. However, I think Popover might be a special case, similar to a modal dialog. Popover appears temporarily on mousehover and doesn't fall within a normal page structure.

There seems to be a lot of debate around what the most appropriate heading level for a modal dialog is but nothing prescriptive.
There's a comment in w3 aria-practices discussion:

It is helpful if there is a consistent approach across dialogs in an application.
There is not necessarily a need for the APG to prescribe a specific approach.

I think we can think of Popover similarly where maybe it's more important to have consistency across Popover. For now, I will leave the :h4 default though we may want to revisit this in the future. Additionally, in dotcom, Popover is actually not accessible at all for keyboard-only users so that is a whole other can of worms we need to address...

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