Deprecate JSON request and responses in favour of SSZ

[bharath-123]

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.