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).~~

consensus/state_processing/src/per_block_processing.rs

@@ -329,10 +329,10 @@ pub fn partially_verify_execution_payload<T: EthSpec>(
         );
     }
     block_verify!(
-        payload.random == *state.get_randao_mix(state.current_epoch())?,
+        payload.prev_randao == *state.get_randao_mix(state.current_epoch())?,
         BlockProcessingError::ExecutionRandaoMismatch {
             expected: *state.get_randao_mix(state.current_epoch())?,
-            found: payload.random,
+            found: payload.prev_randao,
         }
     );
 
@@ -368,7 +368,7 @@ pub fn process_execution_payload<T: EthSpec>(
         state_root: payload.state_root,
         receipts_root: payload.receipts_root,
         logs_bloom: payload.logs_bloom.clone(),
-        random: payload.random,
+        prev_randao: payload.prev_randao,
         block_number: payload.block_number,
         gas_limit: payload.gas_limit,
         gas_used: payload.gas_used,