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.