Warn if using annotation methods which require OpenAPI 3.1 when output mode is set to OpenAPI 3.0

[Azquelt]

New annotation methods were added in MP OpenAPI 4.0 to support the new features added to OpenAPI 3.1. The majority are on @Schema, but there are more across the other annotations (e.g. Info.summary).

When the server is configured to output OpenAPI 3.0, the data contributed by these annotation methods will be dropped at the point that the document is output.

We should warn the user if they've used any of these new fields when the output mode is set to OpenAPI 3.0.

[Azquelt]

I think we should implement this by querying the Jandex index directly, looking for a hard-coded list of annotations and fields and warning if any of them are used.

This does have the downside that we'll only catch use of annotations and not of the model APIs from a reader or filter, but I think that's acceptable since annotations are more commonly used and users using the model directly are likely to have more knowledge of OpenAPI.

Another option would be to scan the model once it's serialized, looking for fields in the model, but this has two issues:

• it will pick up any fields which were generated automatically by the OpenAPI implementation, rather than added deliberately by the user
• it would be impossible to link the problem back to the code that caused it