DEPR: `Styler.where` possible deprecation
closes #40124 

Proposal to deprecate the method: `Styler.where`, since `Styler.applymap` and `Styler.where` are so similar in function, and return signatures, that it is almost confusing to have 2 methods for the same thing.

Note that `Styler.where` offers no advantage over `applymap` since internally it just restructures the callable and uses `applymap` itself:

```
def where(..):
    return self.applymap(lambda val: value if cond(val, **kwargs) else other, subset=subset)
```