validator.swagger.io should not be used as a default

[waschmittel]

I just realized that Swagger UI as included in springdoc-openapi makes requests to validator.swagger.io with the default settings.

This effectively sent URLs of our internal services to validator.swagger.io, which was not what we intended.

Since this was a surprising default (for me), I'd suggest a different one (springdoc.swagger-ui.validator-url: null)

[bnasslahsen]

@waschmittel,

Updated, will be treated as part of #1386