Add alias RemovedAfterNextVersionWarning for deprecations after next version

[Shai Berger]

For a very long time, we've had in django.utils.deprecation something like this (from the 3.2 source code):

class RemovedInDjango40Warning(DeprecationWarning):
    pass


class RemovedInDjango41Warning(PendingDeprecationWarning):
    pass


RemovedInNextVersionWarning = RemovedInDjango40Warning

Or (current master, towards 4.1):

class RemovedInNextVersionWarning(DeprecationWarning):
    pass


class RemovedInDjango50Warning(PendingDeprecationWarning):
    pass

I suggest that we add an alias, RemovedAfterNextVersionWarning for the PendingDeprecationWarning subclass.

The motivation is to make it easier for long-running projects to prepare for future deprecations, with scripts that run tests with special filters on the relevant warnings; currently, such scripts need to be updated for every version. This is especially important for projects which stick to LTS versions, as for them, both warnings need to be treated the same.