Investigate: Why do we have two tags `scroll-output` and `output_scroll` to do the same?

[bsipocz]

Noticed while reviewing a PR https://github.com/executablebooks/MyST-NB/pull/683#discussion_r2183119275, (but didn't notice while reviewing the CSS PR 😱 ) https://github.com/executablebooks/MyST-NB/commit/4b7a0eda8574416aca3e213e8cdaf310427beaf9

I would like to see having only one way to do the same things, but the tags have lived elsewhere before as I have notebooks that use the tags for a few years now, so it's not just a question of making the code here consistent, but also ensuring nothing is breaking.

[bsipocz]

For the tags I would tag @agoose77 as this is something that I suppose is expected to be consistent between the old and new stack.

(and I'm also tagging @dprada as the author of the PR that added all three tags).

[bsipocz]

OK, so after some digging it seems to be that output_scroll has been renamed to be scroll-output a while ago.

See https://github.com/jupyter-book/jupyter-book/issues/1850#issuecomment-1310650823 and linked issues and PRs.

[bsipocz]

Therefore a TODO item here is to cleanup the old one.