The ProblemDetailXmlMapperBuilderCustomizer applied the wrong
ProblemDetail mixin for the Jackson XmlMapper. Namely the
ProblemDetailJacksonMixin, which is intended for JSON.
Now the ProblemDetailJacksonXmlMixin is used instead.
The corresponding test also tested for the wrong XML output,
which has been adjusted.
Fixes gh-48222
Signed-off-by: Maurice Zeijen <mzeijen@bol.com>