add Spring Boot 3 server support, fully tested

[davidmoten]

There was a simple blocker for Spring Boot 3 server generation, being a change to the signature (return type) of ResponseEntity.getStatusCode() between Spring 5 and Spring 6.

This PR adds fully tested support for Spring Boot 3.

Changes include:

• replace artifact openapi-codegen-spring-boot-runtime with Spring Boot version-specific artifacts:
  • openapi-codegen-spring-boot-2-runtime
  • openapi-codegen-spring-boot-3-runtime
• addition of a new module openapi-codegen-spring-boot-core holding shared classes (is depended on by the above two dependencies so you don't need to explicitly add it)
• migrate from slf4j 1.x to 2.x (no apparent problem running Spring Boot 2 test server)
• copy all Spring Boot 2 tests and update for Spring Boot 3 (particularly to use jakarta.servlet-api and multipart support changes).
• add springBoot2 and springBoot3 to accepted generatorTypes in the configuration of openapi-codegen-maven-plugin and deprecated spring2, spring3 (they still work as aliases for springBoot2 and springBoot3)
• trim dependencies (for example javax.annotation only required for Spring Boot 2, and jackson deps were available transitively)
• ensure just junit 5 used (there were a couple of junit 4 references)
• remove unused junit-extras dependency
• update openapi-codegen-pet-store-example project to use Spring Boot 3 (check that out to confirm the appropriate project setup for yourself)

As an aside the server-side code in openapi-codegen-maven-plugin-sb3-test module that demonstrates multipart handling can be simplified because Spring Boot 3 includes multipart features that make life easier. Contribution welcome!

There is only one breaking change for client generation being the move from slf4j 1.x to 2.x.