Add Spring Boot 4.0.0-M2 support

[bnasslahsen]

This feature aims to add support for Spring Boot 4.0.0.

Repos & branches

• Source (Boot 4): springdoc-openapi → spring-boot-4
  https://github.com/springdoc/springdoc-openapi/tree/spring-boot-4
• Docs PRs: springdoc.github.io → update only src/docs/asciidoc/v4
  https://github.com/springdoc/springdoc.github.io
• Demos: springdoc-openapi-demos → 4.x
  https://github.com/springdoc/springdoc-openapi-demos/tree/4.x
• Published docs: https://springdoc.org/v4