[Improvement] application/xml media type instead of text/xml

[tomwetjens]

https://www.rfc-editor.org/rfc/rfc7303 standardizes the media type application/xml for use in exchanging network entities that are related to the Extensible Markup Language (XML) while defining text/xml (...) as aliases for the respective application types.

Also tooling such as DAST scanners mostly support application/xml in the OpenAPI specification.

I propose to change the specification to specify application/xml instead of text/xml.

Producers SHOULD keep sending text/xml until all consumers accept the application/xml type.
Consumers MUST start accepting application/xml and SHOULD accept text/xml for backwards compatibility.

[RobbenRiksen]

Discussed in 2024-03-29 TSC and agreed on. Pull request will be made by @tomwetjens to add this to spec.