Do not use `article` elements without a header

This will need test coverage

Updates to other templates

A change record to announce it.

But also will probably need frontend manager sign off for such a change.

Yes, I would like to get approval for this change before going to all the work of implementing it.

FEFM here:
While I'm not against making these kind of optimizations, something like this could result in regressions on existing sites that have styles/tests/etc that expect <article>. Sites this dependent on specific markup such as this should probably be extending a Stable theme, but I'd still like to be careful that these changes to fundamental templates provide more benefit than risk.

Based on the context in MDN page linked in the IS + reviewing some W3 specs, it seems pretty reasonable to interpret the "should" at face value as opposed to it being a "must". I also see that the default scenario is one where a header appears, and that that default scenario is likely to be true by a very high percentage. Based on the MDN recommendation alone, I'm not sure the regression risk is justified. However, I'm also open to changing my mind - my current reluctance is based only on the info currently available to me. If there's further evidence that this is objectively an accessibility bug, or that there are clear benefits that warrant the regression risk, I'm very much open to hearing that.

An alternative would be to make the template allow setting the wrapper_element in preprocess. If it is not set, use article. This means no change in behaviour, but people can easily make the change if they want without the hassle of overriding the template file.