Update documentation to use STORAGES instead of STATICFILES_STORAGE setting for newer Django versions

[rockstarr-programmerr]

The STATICFILES_STORAGE settings is deprecated in Django 4.2 and officially removed in Django 5.1

Not updating the documentation can lead to confusion for users of Django 5.1, as it will appear that django-pipeline simply doesn't do anything when the user follow the instruction and set STATICFILES_STORAGE instead of STORAGES.

I suggest updating the documentation to showcase using STATICFILES_STORAGE for Django < 4.2 and STORAGES for Django >= 4.2, for example:

For Django < 4.2:

STATICFILES_STORAGE = 'pipeline.storage.PipelineManifestStorage'

For Django >= 4.2:

STORAGES = {
    'default': {
        'BACKEND': <your default storage backend>,  # Django's default is 'django.core.files.storage.FileSystemStorage'
    },
    'staticfiles': {
        'BACKEND': 'pipeline.storage.PipelineManifestStorage',
    },
}