Set `_format` to `json` on most routes

Problem/Motivation

I've been working on getting better exception handling on non-HTML routes and non-404 exceptions, similar to what JSON:API module does for its paths. That led me to creating #3304981: JSON and other serialization formats now handle non-4xx exceptions → . This is particularly helpful when implementing extended grant types (e.g., token exchange) that have to do a lot of backend processing and might be more likely to throw an exception due to non-OAuth specific reasons.

We should mark our routes that only speak JSON for their request or response format accordingly so that code like this exception handling issue can know what format the route speaks, without the client needing to set ?_format=json.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes