Consider FutureWarning instead of DeprecationWatning

[bryevdv]

Someone at work today mentioned:

Also, I would recommend using FutureWarning instead of DeprecationWarning.... The upshot is that DeprecationWarning is really meant for Python internal use because Python hides it by default, whereas FutureWarning is visible to users and is actionable

I have not actually looked into this yet, but jotting it down so I don't forget. If this is really the case then I think we could jettison most or all of our custom logging code that tries to force deprecation warnings to be visible, which would be terrific.

cc @bokeh/dev

[bryevdv]

Just noting the Python docs for FutureWarning state

Base class for warnings about deprecated features when those warnings are intended for end users of applications that are written in Python.

This is in contrast with DeprecationWarning which is intended "for other Python developers" (which is why it is hidden by default). I think it's fair to say that our user base is a mix of both "end users" and "other developers" in the sense those docs intend, but I think it's also fair to say that the majority would be considered "end users", so that switching to FutureWarning would be reasonable.