Remove spring-boot-json-test module and spread code between
`spring-boot-test`, `spring-boot-test-autoconfigure` and JSON
technology modules.
See gh-46356
See gh-47322
Prior to this commit, Spring Boot would use Framework's
`Jackson2ObjectMapperBuilder` to configure the `ObjectMapper` instance.
This builder would configure the `ProblemDetail` mixin automatically.
With the introduction of Jackson 3.x support, Spring Framework removed
its builder in favor of the native Jackson builder. As a result, the
mixin is not registered with the `JsonMapper` aymore.
This commit ensures that the mixin is registered if the `ProblemDetail`
class is present in the classpath.
Closes gh-47298
Andy Wilkinson
Moritz Halbritter
Phillip Webb
Brian Clozel