[java][jersey2] remove warning using JsonMapper.builder

[wing328]

• remove warning using JsonMapper.builder

[WARNING] /Users/williamcheng/Code/openapi-generator/samples/openapi3/client/petstore/java/jersey2-java8/src/main/java/org/openapitools/client/JSON.java:[24,10] [deprecation] configure(MapperFeature,boolean) in ObjectMapper has been deprecated

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[snago]

I noticed this line in a generated client:
JsonMapper.builder().configure(MapperFeature.ALLOW_COERCION_OF_SCALARS, false);
and the history led me to this PR.

That line creates a builder, sets a config and then throws the builder away. Totally non-sensical.

To use a builder it should look something like this:

    mapper = JsonMapper.builder()
                       .serializationInclusion(JsonInclude.Include.NON_NULL)
                       .configure(MapperFeature.ALLOW_COERCION_OF_SCALARS, false)
                       .configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, true)
                       .configure(DeserializationFeature.FAIL_ON_INVALID_SUBTYPE, true)
                       .disable(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS)
                       .enable(SerializationFeature.WRITE_ENUMS_USING_TO_STRING)
                       .enable(DeserializationFeature.READ_ENUMS_USING_TO_STRING)
                       .defaultDateFormat(new RFC3339DateFormat())
                       .addModule(new JavaTimeModule())
                       .addModule(new JsonNullableModule())
                       .build();