Deprecate support for registering a `PluginTemplateExtension` subclass via `model` (singular)

[jeremystretch]

Proposed Changes

Deprecate support for the singular model attribute on PluginTemplateExtension. Subclasses will need to update accordingly:

Old

class MyExtension(PluginTemplateExtension):
    model = 'dcim.site'

New

class MyExtension(PluginTemplateExtension):
    models = ['dcim.site']

Justification

NetBox v4.1 introduced support for registering on one or more models via the new models (plural) attribute, which obsoleted the singular form.

Impact

PluginTemplateExtension subclasses which continue to register using the model attribute will generate a deprecation warning.