COMPAT: Update old numpy aliases
- [x] closes #55454 (Replace xxxx with the GitHub issue number)
- [ ] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature


@lithomas1 do you think we should backport this to 2.1.2? It appears that Numpy 2.0 is going to remove the `np.dtype("a")` alias https://github.com/numpy/numpy/pull/24854 ~~We'd need the minimum version of Cython to be 3.0 to be fully compatible with numpy 2.0 and I'm not sure if that's going to happen before we release 2.2.
It might be better to just focus on making main(2.2) work with numpy 2.0 isntead, given we will release that in a month or two anyways.~~

EDIT: I changed my mind. Even if 2.1.x doesn't end up fully compatible with numpy 2.0, it'd still be good to have this in, to not have red CI there, and this is harmless enough. thanks @mroeschke. @meeseeksdev backport 2.1.x. @meeseeksdev backport 2.1.x.