Drop backward compatibility for importing plugin resources from `extras` app

[jeremystretch]

Proposed Changes

Plugin resources were moved from the extras app to the netbox app in v3.7, while retaining backward compatibility to avoid a breaking change. This issue exists to track the removal of this backward compatibility in the v4.0 release.

Plugins which have not already adapted to the refactoring will need to update their imports. For example:

from extras.plugins import PluginConfig

becomes

from netbox.plugins import PluginConfig

Justification

Planned deprecation