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/rpc/protocol.rs

@@ -57,7 +57,7 @@ pub static SIGNED_BEACON_BLOCK_ALTAIR_MAX: LazyLock<usize> = LazyLock::new(|| {
 /// The `BeaconBlockBellatrix` block has an `ExecutionPayload` field which has a max size ~16 GiB for future proofing.
 /// We calculate the value from its fields instead of constructing the block and checking the length.
 /// Note: This is only the theoretical upper bound. We further bound the max size we receive over the network
-/// with `max_chunk_size`.
+/// with `max_payload_size`.
 pub static SIGNED_BEACON_BLOCK_BELLATRIX_MAX: LazyLock<usize> =
     LazyLock::new(||     // Size of a full altair block
     *SIGNED_BEACON_BLOCK_ALTAIR_MAX
@@ -122,15 +122,6 @@ const PROTOCOL_PREFIX: &str = "/eth2/beacon_chain/req";
 /// established before the stream is terminated.
 const REQUEST_TIMEOUT: u64 = 15;
 
-/// Returns the maximum bytes that can be sent across the RPC.
-pub fn max_rpc_size(fork_context: &ForkContext, max_chunk_size: usize) -> usize {
-    if fork_context.current_fork().bellatrix_enabled() {
-        max_chunk_size
-    } else {
-        max_chunk_size / 10
-    }
-}
-
 /// Returns the rpc limits for beacon_block_by_range and beacon_block_by_root responses.
 ///
 /// Note: This function should take care to return the min/max limits accounting for all