message: Also check for downgrades when fetching the trusted key block.
diff: @@ -239,6 +239,7 @@ async fn get_trusted_key_block_info(
  if let Some(key_block_info) =
  KeyBlockInfo::maybe_from_block_header(&current_header_to_walk_back_from)
  {
  + check_block_version(trusted_header, &key_block_info, chainspec)?;
  break Ok(key_block_info);
  }
  @@ -506,9 +507,8 @@ async fn fast_sync_to_most_recent(
  trusted_key_block_info = key_block_info;
  }
  }
  - // If we could not fetch, we can stop when the most recent header:
  - // 1. has our protocol version
  - // 2. is in the current era
  + // If we could not fetch, we can stop when the most recent header is in the current
  + // era.
  None if is_current_era(
  &most_recent_block_header,
  &trusted_key_block_info,
  