Implement el_offline and use it in the VC (#4295)

## Issue Addressed

Closes https://github.com/sigp/lighthouse/issues/4291, part of #3613.

## Proposed Changes

- Implement the `el_offline` field on `/eth/v1/node/syncing`. We set `el_offline=true` if:
  - The EL's internal status is `Offline` or `AuthFailed`, _or_
  - The most recent call to `newPayload` resulted in an error (more on this in a moment).

- Use the `el_offline` field in the VC to mark nodes with offline ELs as _unsynced_. These nodes will still be used, but only after synced nodes.
- Overhaul the usage of `RequireSynced` so that `::No` is used almost everywhere. The `--allow-unsynced` flag was broken and had the opposite effect to intended, so it has been deprecated.
- Add tests for the EL being offline on the upcheck call, and being offline due to the newPayload check.


## Why track `newPayload` errors?

Tracking the EL's online/offline status is too coarse-grained to be useful in practice, because:

- If the EL is timing out to some calls, it's unlikely to timeout on the `upcheck` call, which is _just_ `eth_syncing`. Every failed call is followed by an upcheck [here](693886b941/beacon_node/execution_layer/src/engines.rs (L372-L380)), which would have the effect of masking the failure and keeping the status _online_.
- The `newPayload` call is the most likely to time out. It's the call in which ELs tend to do most of their work (often 1-2 seconds), with `forkchoiceUpdated` usually returning much faster (<50ms).
- If `newPayload` is failing consistently (e.g. timing out) then this is a good indication that either the node's EL is in trouble, or the network as a whole is. In the first case validator clients _should_ prefer other BNs if they have one available. In the second case, all of their BNs will likely report `el_offline` and they'll just have to proceed with trying to use them.

## Additional Changes

- Add utility method `ForkName::latest` which is quite convenient for test writing, but probably other things too.
- Delete some stale comments from when we used to support multiple execution nodes.

validator_client/src/check_synced.rs

@@ -36,7 +36,10 @@ pub async fn check_synced<T: SlotClock>(
         }
     };
 
-    let is_synced = !resp.data.is_syncing || (resp.data.sync_distance.as_u64() < SYNC_TOLERANCE);
+    // Default EL status to "online" for backwards-compatibility with BNs that don't include it.
+    let el_offline = resp.data.el_offline.unwrap_or(false);
+    let bn_is_synced = !resp.data.is_syncing || (resp.data.sync_distance.as_u64() < SYNC_TOLERANCE);
+    let is_synced = bn_is_synced && !el_offline;
 
     if let Some(log) = log_opt {
         if !is_synced {
@@ -52,6 +55,7 @@ pub async fn check_synced<T: SlotClock>(
                 "sync_distance" => resp.data.sync_distance.as_u64(),
                 "head_slot" => resp.data.head_slot.as_u64(),
                 "endpoint" => %beacon_node,
+                "el_offline" => el_offline,
             );
         }