Revisit Actuator MediaTypeVersion handling

[TimHess]

During recent trace actuator work, we discovered that the Content-Type header in actuator responses is not being handled correctly. It might make sense for individual actuator endpoints to be able to specify their media type (eg: /httpExchanges should only ever return v3) and not exclusively respond to an Accepts header and falling back to v2 if the header wasn't found.

We should investigate if any actuator front-ends we care about even pay attention to this header, and if so, how Spring determines the media type to use in the response.

[TimHess]

This issue and #1335 should be addressed at the same time

[bart-vmware]

Related: it's currently not possible to change the version of the health actuator, because both HealthConverter and HealthConverterV3 are internal.