fix: make topk() use {name}_count format, like .value_counts()

[NickCrews]

Fixes (partially) https://github.com/ibis-project/ibis/issues/8540

this is breaking, but I think much better.

[cpcloud]

We are going to do a point release soon, and since this is a breaking change it won't go into that release. This will need to go into the next breaking release which will be a while given we have a number of other things to do.

[NickCrews]

That sounds fine to me. Just keep this open and merge it in once were ready?

[cpcloud]

Closing this out. As of 9.5 you can pass your own name to the name parameter of value_counts and topk.