NetBox 4.3 and up prefer STORAGES to STORAGE_BACKEND

[sleinen]

Since netbox-community/netbox#18680 (landed in NetBox 4.3), remote storage services should be configured using Django's STORAGES configuration option rather than using STORAGE_BACKEND. The logic in tasks/validate_variables.yml only looks at STORAGE_BACKEND, and derives _netbox_storages_module from it. I think it should be generalized to extract possibly multiple storage modules from STORAGES (and maybe fall back to STORAGE_BACKEND if STORAGES is not set, possibly with a warning if STORAGE_BACKEND is set for NetBox version 4.3 or greater, because that will not work).