Previously, @AutoConfigureJsonTesters only imported
JsonTestersAutoConfiguration and relied on something else pulling in
the Jackson and GSON auto-configuration upon which it depends. This
worked with @JsonTest which imported those auto-configurations. It
did not work with @SpringBootTest which would use
@EnableAutoConfiguration and the ordering was then wrong and
JsonTestersAutoConfiguration would be processed before the Jackson
and GSON auto-configurations had a chance to create the beans that
JsonTestersAutoConfiguration needs.

This commit updates the spring.factories configuration for
JsonTestersAutoConfiguration so that it imports
JacksonAutoConfiguration and GsonAutoConfiguration.
Appropriate @AutoConfigureAfter has also been added to
JsonTestersAutoConfiguration to ensure that it is considered after
JacksonAutoConfiguration and GsonAutoConfiguration.

Lastly, ExampleJsonApplication and associated classes have been moved
into an app sub-package to prevent its component scanning from
pulling in JsonTestersAutoConfiguration as if it were user
configuration.

Closes gh-9515