Unpin `category-encoders` dependency

In #3933, we ended up pinning the `category-encoders` dependency as it kept returning ~0.6 for the `TargetEncoder()` component when [tests are ran on main](https://github.com/alteryx/evalml/actions/runs/3939944955/jobs/6740422467).

This issue tracks finding the root cause of this regression and resolving it so that the current test cases are functional again.