Remove single-model registration from PluginTemplateExtension

[jeremystretch]

Proposed Changes

Drop support for registering a single model with a PluginTemplateExtension subclass via the model attribute.

Justification

Model assignment is now done through the models (plural) attribute, which accepts an iterable of one or more models (see #17029)

Impact

Any plugins which employ PluginTemplateExtension subclasses will need to be slightly modified to support NetBox v4.3. See #17029 for implementation details.