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/duties_service.rs

@@ -147,11 +147,6 @@ pub struct DutiesService<T, E: EthSpec> {
     pub slot_clock: T,
     /// Provides HTTP access to remote beacon nodes.
     pub beacon_nodes: Arc<BeaconNodeFallback<T, E>>,
-    /// Controls whether or not this function will refuse to interact with non-synced beacon nodes.
-    ///
-    /// This functionality is a little redundant since most BNs will likely reject duties when they
-    /// aren't synced, but we keep it around for an emergency.
-    pub require_synced: RequireSynced,
     pub enable_high_validator_count_metrics: bool,
     pub context: RuntimeContext<E>,
     pub spec: ChainSpec,
@@ -421,7 +416,7 @@ async fn poll_validator_indices<T: SlotClock + 'static, E: EthSpec>(
             let download_result = duties_service
                 .beacon_nodes
                 .first_success(
-                    duties_service.require_synced,
+                    RequireSynced::No,
                     OfflineOnFailure::Yes,
                     |beacon_node| async move {
                         let _timer = metrics::start_timer_vec(
@@ -618,7 +613,7 @@ async fn poll_beacon_attesters<T: SlotClock + 'static, E: EthSpec>(
         if let Err(e) = duties_service
             .beacon_nodes
             .run(
-                duties_service.require_synced,
+                RequireSynced::No,
                 OfflineOnFailure::Yes,
                 |beacon_node| async move {
                     let _timer = metrics::start_timer_vec(
@@ -856,7 +851,7 @@ async fn post_validator_duties_attester<T: SlotClock + 'static, E: EthSpec>(
     duties_service
         .beacon_nodes
         .first_success(
-            duties_service.require_synced,
+            RequireSynced::No,
             OfflineOnFailure::Yes,
             |beacon_node| async move {
                 let _timer = metrics::start_timer_vec(
@@ -1063,7 +1058,7 @@ async fn poll_beacon_proposers<T: SlotClock + 'static, E: EthSpec>(
         let download_result = duties_service
             .beacon_nodes
             .first_success(
-                duties_service.require_synced,
+                RequireSynced::No,
                 OfflineOnFailure::Yes,
                 |beacon_node| async move {
                     let _timer = metrics::start_timer_vec(