[BUG][Java] required fields are not validated

Description

Currently when I define required fields in the api.yaml, they are not validated in the generated classes.
It returns a HTTP 200 OK and proceeds, instead of returning HTTP 405 invalid input

Actual:

you can find the example project here: https://github.com/bobocu/openapi-demo

POST localhost:8091/api/v1/pet
body: { "test": "test" }

return HTTP 200 OK and defined log message is displayed in sysout.

openapi-generator version

5.1.0

OpenAPI declaration file content or url

just use the default petstore yaml
https://github.com/bobocu/openapi-demo/blob/master/src/main/resources/static/api.yaml

Generation Details

execute openApiGenearte
import spring boot gradle project and start on local machine

Steps to reproduce

execute POST request to localhost:8091/api/v1/pet
with various json body

Start with the default Petstore api.yaml and the example Spring Boot Gradle project at bobocu/openapi-demo, then reproduce POST /api/v1/pet with the incomplete JSON body. Inspect the generated classes and request handling; the work is done when required fields are validated and the request no longer returns HTTP 200 OK.