Specify _format as json for all XB HTTP API routes

Making minor

jeeva r → made their first commit to this issue’s fork.

please review https://git.drupalcode.org/project/experience_builder/-/merge_requests/1087

Pragmatic. Let’s land it!

P.S.: there’s issues (in this same issue queue component, not linking because writing from phone) that aim to actually use Drupal’s/Symfony’s serialization formats (so: normalizers), but even then this is accurate. All that will do is create a specific subtype of the JSON format: application/json+xb or something like it.