Rename random to prev_randao (#3040)

## Issue Addressed

As discussed on last-night's consensus call, the testnets next week will target the [Kiln Spec v2](https://hackmd.io/@n0ble/kiln-spec).

Presently, we support Kiln V1. V2 is backwards compatible, except for renaming `random` to `prev_randao` in:

- https://github.com/ethereum/execution-apis/pull/180
- https://github.com/ethereum/consensus-specs/pull/2835

With this PR we'll no longer be compatible with the existing Kintsugi and Kiln testnets, however we'll be ready for the testnets next week. I raised this breaking change in the call last night, we are all keen to move forward and break things.

We now target the [`merge-kiln-v2`](https://github.com/MariusVanDerWijden/go-ethereum/tree/merge-kiln-v2) branch for interop with Geth. This required adding the `--http.aauthport` to the tester to avoid a port conflict at startup.

### Changes to exec integration tests

There's some change in the `merge-kiln-v2` version of Geth that means it can't compile on a vanilla Github runner. Bumping the `go` version on the runner solved this issue.

Whilst addressing this, I refactored the `testing/execution_integration` crate to be a *binary* rather than a *library* with tests. This means that we don't need to run the `build.rs` and build Geth whenever someone runs `make lint` or `make test-release`. This is nice for everyday users, but it's also nice for CI so that we can have a specific runner for these tests and we don't need to ensure *all* runners support everything required to build all execution clients.

## More Info

- [x] ~~EF tests are failing since the rename has broken some tests that reference the old field name. I have been told there will be new tests released in the coming days (25/02/22 or 26/02/22).~~

beacon_node/execution_layer/src/engines.rs

@@ -50,7 +50,7 @@ impl Logging {
 struct PayloadIdCacheKey {
     pub head_block_hash: ExecutionBlockHash,
     pub timestamp: u64,
-    pub random: Hash256,
+    pub prev_randao: Hash256,
     pub suggested_fee_recipient: Address,
 }
 
@@ -77,7 +77,7 @@ impl<T> Engine<T> {
         &self,
         head_block_hash: ExecutionBlockHash,
         timestamp: u64,
-        random: Hash256,
+        prev_randao: Hash256,
         suggested_fee_recipient: Address,
     ) -> Option<PayloadId> {
         self.payload_id_cache
@@ -86,7 +86,7 @@ impl<T> Engine<T> {
             .get(&PayloadIdCacheKey {
                 head_block_hash,
                 timestamp,
-                random,
+                prev_randao,
                 suggested_fee_recipient,
             })
             .cloned()
@@ -393,7 +393,7 @@ impl PayloadIdCacheKey {
         Self {
             head_block_hash: state.head_block_hash,
             timestamp: attributes.timestamp,
-            random: attributes.random,
+            prev_randao: attributes.prev_randao,
             suggested_fee_recipient: attributes.suggested_fee_recipient,
         }
     }