Clarify network limits (#7175)

Resolves #6811


  Rename `GOSSIP_MAX_SIZE` to `MAX_PAYLOAD_SIZE` and remove `MAX_CHUNK_SIZE` in accordance with the spec.

The spec also "clarifies"  the message size limits at different levels. The rpc limits are equivalent to what we had before imo.
The gossip limits have additional checks.

I have gotten rid of the `is_bellatrix_enabled`  checks that used a lower limit (1mb) pre-merge. Since all networks we run start from the merge, I don't think this will break any setups.

beacon_node/lighthouse_network/src/lib.rs

@@ -12,7 +12,6 @@ pub mod peer_manager;
 pub mod rpc;
 pub mod types;
 
-pub use config::gossip_max_size;
 use libp2p::swarm::DialError;
 pub use listen_addr::*;