[BUG] AsciidocDocumentationCodegen does not properly use `appDescription` additional property

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

The asciidoc output always contains No description provided (generated by Openapi Generator https://github.com/openapitools/openapi-generator), setting the additional property appDescription has no impact on the resulting asciidoc.

openapi-generator version

5.0.0-SNAPSHOT

OpenAPI declaration file content or url

Generation Details

Steps to reproduce

Use asciidoc output

docker run --rm -u $(id -u) -v $(pwd)/output:/local openapitools/openapi-generator-cli generate -g asciidoc -c /local/openapi-properties.yaml -i /local/api.json -o /local/ 

Related issues/PRs

Suggest a fix

The codegen https://github.com/OpenAPITools/openapi-generator/blob/master/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/AsciidocDocumentationCodegen.java does not seem to use the property.

Start with modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/AsciidocDocumentationCodegen.java and reproduce the issue using the Docker command in the report with an appDescription additional property. Compare the generated Asciidoc output with the expected description behavior; done means appDescription affects the output instead of the default message.