Remove support for the #value attribute of the #type = details element

Updated: Comment #0

Problem/Motivation

Over in #504962: Provide a compound form element with accessible labels @effulgentsia removed support for #value attribute of #type = details, as we already have #children.
However, this broke a few modules including views ui, so there are places in core that use #value with #type details.

Proposed resolution

Remove #value support from #type details, child elements should be just that... child elements.
Fix instances in core that use #value.

Remaining tasks

Patch
Tests already exist so we will get immediate feedback.

User interface changes

None

API changes

No longer support #value for #type details, although arguably this should not have existed in the first place.