CLN: Enforce deprecation of PeriodIndex in resample

[rhshadrach]

• closes #xxxx (Replace xxxx with the GitHub issue number)
• Tests added and passed if fixing a bug or adding a new feature
• All code checks passed.
• Added type annotations to new arguments/methods/functions.
• Added an entry in the latest doc/source/whatsnew/vX.X.X.rst file if fixing a bug or adding a new feature.

[jbrockmendel]

Pls hold off on this one. Will explain why when I get to a keyboard

[jbrockmendel]

For the curious: resample with PeriodIndex was deprecated bc a bunch of it is pretty badly broken (see #53481) and converting to DatetimeIndex before resampling should work fine. But #57033 makes it seem like there are cases where converting to DTI isn't a valid alternative. (I suspect that in these cases the "resample" is doing something meaningfully distinct that should have a different name, but don't hold me to that).

So we're holding off on enforcing the deprecation, but not reverting it until/unless someone steps up to be the champion for fixing/maintaining the broken bits.

[rhshadrach]

Thanks for the context here, I'll adjust some of the commenting around the deprecation in the code.