Introduce getModelSchema and soft-deprecate getModelSchemaRef

[dhmlau]

Opening a follow up issue according to: #4328 (comment)

Cross posting @bajtos' comment from the above link:

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After we land feat(rest): add openapi schema consolidation #4365, I'd like to start the process of deprecating getModelSchemaRef and reworking both our existing code and CLI templates to leverage the new schema consolidation:

• Introduce getModelSchema similar to getModelSchemaRef (see src/controller-spec.ts)
• Rework our controller templates to use getModelSchema instead of getModelSchemaRef
• Rework controllers inside /examples to use getModelSchema
• Soft-deprecate getModelSchemaRef in our documentation (both /docs files and the tsdoc comment)
• Update https://github.com/strongloop/loopback4-example-shopping to use getModelSchema

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cc @bajtos @jannyHou