Deprecate step param in BlocksByRange RPC request (#3275)

## Issue Addressed

Deprecates the step parameter in the blocks by range request

## Proposed Changes

- Modifies the BlocksByRangeRequest type to remove the step parameter and everywhere we took it into account before
- Adds a new type to still handle coding and decoding of requests that use the parameter

## Additional Info
I went with a deprecation over the type itself so that requests received outside `lighthouse_network` don't even need to deal with this parameter. After the deprecation period just removing the Old blocks by range request should be straightforward

beacon_node/lighthouse_network/src/rpc/protocol.rs

@@ -279,8 +279,8 @@ impl ProtocolId {
                 <GoodbyeReason as Encode>::ssz_fixed_len(),
             ),
             Protocol::BlocksByRange => RpcLimits::new(
-                <BlocksByRangeRequest as Encode>::ssz_fixed_len(),
-                <BlocksByRangeRequest as Encode>::ssz_fixed_len(),
+                <OldBlocksByRangeRequest as Encode>::ssz_fixed_len(),
+                <OldBlocksByRangeRequest as Encode>::ssz_fixed_len(),
             ),
             Protocol::BlocksByRoot => {
                 RpcLimits::new(*BLOCKS_BY_ROOT_REQUEST_MIN, *BLOCKS_BY_ROOT_REQUEST_MAX)
@@ -415,7 +415,7 @@ where
 pub enum InboundRequest<TSpec: EthSpec> {
     Status(StatusMessage),
     Goodbye(GoodbyeReason),
-    BlocksByRange(BlocksByRangeRequest),
+    BlocksByRange(OldBlocksByRangeRequest),
     BlocksByRoot(BlocksByRootRequest),
     Ping(Ping),
     MetaData(PhantomData<TSpec>),