Add obsolete guidance for OAS 3.2 deprecated properties

[baywet]

Context

OAS 3.2.0 marks several fields/features as deprecated. This issue tracks the OpenAPI.NET object model updates needed for deprecated specification properties, while intentionally excluding the deprecated boolean flags that users can set on individual OpenAPI description instances.

Specification: https://spec.openapis.org/oas/v3.2.0.html

Excluded from model obsoletion

Do not add [Obsolete] for the deprecated boolean properties themselves. These remain valid user-authored flags for indicating an instance is deprecated:

Object	Property
Operation Object	deprecated
Parameter Object	deprecated
Header Object	deprecated
Security Scheme Object	deprecated

Properties to mark obsolete

Add [Obsolete] attributes to guide users toward the updated replacements:

Object	Deprecated property	Guidance / replacement
Parameter Object	allowEmptyValue	Use is not recommended by OAS 3.2.0 and it is likely to be removed in a later revision.
Schema Object	example	Use JSON Schema examples instead.
XML Object	attribute	Use nodeType: "attribute" instead.
XML Object	wrapped	Use nodeType: "element" instead.

Conditional deprecation note

The Example Object value field is deprecated only for non-JSON serialization targets. Since the deprecation is conditional, updating the object model would create noise for scenarios where value is still valid. Do not add [Obsolete] for this property.

Acceptance criteria

• Deprecated OAS 3.2.0 properties listed above have [Obsolete] attributes in the object model with actionable replacement guidance.
• Boolean deprecated instance flags are left unchanged.
• Example Object value is left unchanged, with no object model obsoletion.