Activate clippy::manual_let_else lint (#4889)

## Issue Addressed

#4888

## Proposed Changes

Enabled `clippy::manual_let_else` lint and resolved the warning messages.

beacon_node/lighthouse_network/src/rpc/codec/ssz_snappy.rs

@@ -135,9 +135,8 @@ impl<TSpec: EthSpec> Decoder for SSZSnappyInboundCodec<TSpec> {
         if self.protocol.versioned_protocol == SupportedProtocol::MetaDataV2 {
             return Ok(Some(InboundRequest::MetaData(MetadataRequest::new_v2())));
         }
-        let length = match handle_length(&mut self.inner, &mut self.len, src)? {
-            Some(len) => len,
-            None => return Ok(None),
+        let Some(length) = handle_length(&mut self.inner, &mut self.len, src)? else {
+            return Ok(None);
         };
 
         // Should not attempt to decode rpc chunks with `length > max_packet_size` or not within bounds of
@@ -277,9 +276,8 @@ impl<TSpec: EthSpec> Decoder for SSZSnappyOutboundCodec<TSpec> {
                 return Ok(None);
             }
         }
-        let length = match handle_length(&mut self.inner, &mut self.len, src)? {
-            Some(len) => len,
-            None => return Ok(None),
+        let Some(length) = handle_length(&mut self.inner, &mut self.len, src)? else {
+            return Ok(None);
         };
 
         // Should not attempt to decode rpc chunks with `length > max_packet_size` or not within bounds of
@@ -324,9 +322,8 @@ impl<TSpec: EthSpec> OutboundCodec<OutboundRequest<TSpec>> for SSZSnappyOutbound
         &mut self,
         src: &mut BytesMut,
     ) -> Result<Option<Self::CodecErrorType>, RPCError> {
-        let length = match handle_length(&mut self.inner, &mut self.len, src)? {
-            Some(len) => len,
-            None => return Ok(None),
+        let Some(length) = handle_length(&mut self.inner, &mut self.len, src)? else {
+            return Ok(None);
         };
 
         // Should not attempt to decode rpc chunks with `length > max_packet_size` or not within bounds of

beacon_node/lighthouse_network/src/rpc/handler.rs

@@ -286,9 +286,7 @@ where
     // wrong state a response will fail silently.
     fn send_response(&mut self, inbound_id: SubstreamId, response: RPCCodedResponse<TSpec>) {
         // check if the stream matching the response still exists
-        let inbound_info = if let Some(info) = self.inbound_substreams.get_mut(&inbound_id) {
-            info
-        } else {
+        let Some(inbound_info) = self.inbound_substreams.get_mut(&inbound_id) else {
             if !matches!(response, RPCCodedResponse::StreamTermination(..)) {
                 // the stream is closed after sending the expected number of responses
                 trace!(self.log, "Inbound stream has expired. Response not sent";
@@ -296,7 +294,6 @@ where
             }
             return;
         };
-
         // If the response we are sending is an error, report back for handling
         if let RPCCodedResponse::Error(ref code, ref reason) = response {
             self.events_out.push(Err(HandlerErr::Inbound {

beacon_node/lighthouse_network/src/service/gossip_cache.rs

@@ -205,9 +205,8 @@ impl GossipCache {
             GossipKind::LightClientFinalityUpdate => self.light_client_finality_update,
             GossipKind::LightClientOptimisticUpdate => self.light_client_optimistic_update,
         };
-        let expire_timeout = match expire_timeout {
-            Some(expire_timeout) => expire_timeout,
-            None => return,
+        let Some(expire_timeout) = expire_timeout else {
+            return;
         };
         match self
             .topic_msgs