Make broadcasting over sets an error

[mbauman]

Ref #23746.

[vtjnash]

I don't think we should be adding random special cases here. For one thing, there exist OrderedSets – and we're even considering specifying them as the default in base (#10116), when we get some time to finish testing – so I disagree that it should require a special case here banning all AbstractSets.

Even more importantly, even though the ordering is undefined, it is guaranteed to be stable, so there can be cases where it would make sense to use broadcast where one of the iterable arguments happens to be a Set. An arbitrary artificial example is:

make_row(k, v, av) -> (pk=k, value=v, absvalue=av)
dictionary_to_table(dictionary) = make_row.(keys(dictionary), values(dictionary), abs.(values(dictionary)))

[mbauman]

Ah, phew — good! Also, I found #26980 this morning, which I had missed last night. The plan I enumerated there is exactly the status quo on master right now.

I just wanted to be as conservative in the deprecation removal process as is reasonable.