Previously, the actuator's endpoints produced application/json and,
where appropriate, also consumed application/json. Without a custom,
versioned media type, it's impossible for us to make changes to the
endpoints without breaking clients.

This commit introduces a new media type,
application/spring-boot.actuator.v1+json, that is now produced by
default with application/json also being produced if requested.
Endpoints that consume JSON will now also accept content the uses
the new media type in addition to application/json.

Closes gh-7967