Rollup merge of #81500 - CraftSpider:union-kind, r=jyn514

Rollup merge of #81500 - CraftSpider:union-kind, r=jyn514

Remove struct_type from union output

Also bumps the format number and adds a test

Rationale: It's illegal to have unions of the form `union Union(i32, f32);`, or `union Union;`. The struct_type field was recently removed from the rustdoc Union AST, at which time this field was changed to always just read "union". It makes sense to completely remove it, as it provides no information.