Generator: Add deprecated support

[TimAufdemKampe]

If I mark a field of my api spec as deprecated: true my spec get invalid.

properties:
    lastMessageAt:
        type: string
        format: date-time
        deprecated: true

Docs: https://openapispec.com/docs/how/how-does-openapi-handle-api-deprecation/#deprecation-of-schema-properties

[martin-helmich]

This has been laying around for a while, but: Can you describe this more closely? What does "get invalid" entail, and how does it affect client generation?