Deprecate JSON request and responses in favour of SSZ

SSZ serialization does a better job reducing response/request sizes over the wire compared to JSON.
Currently, relays and builders support both SSZ and JSON serialized objects based on the `Content-Type` header.
It would be much cleaner if we support only SSZ serialization post the fulu hard fork.

What would be the best strategy to deprecate JSON in favour of SSZ?
We could have the following strategies:

1. Completely remove JSON support for Fulu and allow only SSZ serde.
2. Put up a deprecation notice for Fulu but support both JSON and SSZ. We can remove JSON and SSZ support at the first release post Fulu.

Start by reviewing the builder-specs requirements for relay and builder serialization around the Fulu hard fork, especially the Content-Type behavior described here. The issue is complete when the project has chosen and documented a deprecation strategy for JSON versus SSZ; no files or tests are identified in the payload.

Written by the indexing model from the issue text.