Add 'as' to FieldContainer

[gwyneplaine]

Currently there are quite a few places where we use legend elements to denote field-groups.
None of these are working because the FieldContainer parent is a div, not a fieldset.

This is a minor patch to our ds component, in preparation for incoming a11y fixes in fields.

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Latest commit: ac1e341

The changes in this PR will be included in the next version bump.

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Name	Type
@keystone-ui/fields	Minor

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