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/outbound.rs

@@ -36,7 +36,7 @@ pub struct OutboundRequestContainer<TSpec: EthSpec> {
 pub enum OutboundRequest<TSpec: EthSpec> {
     Status(StatusMessage),
     Goodbye(GoodbyeReason),
-    BlocksByRange(BlocksByRangeRequest),
+    BlocksByRange(OldBlocksByRangeRequest),
     BlocksByRoot(BlocksByRootRequest),
     Ping(Ping),
     MetaData(PhantomData<TSpec>),