feat(python): split Expr.top_k and Expr.top_k_by into separate functions

[MarcoGorelli]

closes #16037

Summary of changes:

• top_k_by is broken off from top_k
• removed by and descending args from top_k (they aren't in the latest stable release. by is being removed to split out top_k_by, and descending just doesn't seem useful for a single top_k, you can just use bottom_k...)
• drive-by: corrected log{}n to log{n} in the docstring
• removed some tests which are no longer relevant (that's one advantage of splitting the functions up actually)

(same for bottom_k / bottom_k_by)

[ritchie46]

Nice, now the release isn't blocked 😁