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).~~

testing/execution_engine_integration/src/test_rig.rs

@@ -138,7 +138,7 @@ impl<E: GenericExecutionEngine> TestRig<E> {
 
         let parent_hash = terminal_pow_block_hash;
         let timestamp = timestamp_now();
-        let random = Hash256::zero();
+        let prev_randao = Hash256::zero();
         let finalized_block_hash = ExecutionBlockHash::zero();
         let proposer_index = 0;
         let valid_payload = self
@@ -147,7 +147,7 @@ impl<E: GenericExecutionEngine> TestRig<E> {
             .get_payload::<MainnetEthSpec>(
                 parent_hash,
                 timestamp,
-                random,
+                prev_randao,
                 finalized_block_hash,
                 proposer_index,
             )
@@ -210,7 +210,7 @@ impl<E: GenericExecutionEngine> TestRig<E> {
          */
 
         let mut invalid_payload = valid_payload.clone();
-        invalid_payload.random = Hash256::from_low_u64_be(42);
+        invalid_payload.prev_randao = Hash256::from_low_u64_be(42);
         let status = self
             .ee_a
             .execution_layer
@@ -227,7 +227,7 @@ impl<E: GenericExecutionEngine> TestRig<E> {
 
         let parent_hash = valid_payload.block_hash;
         let timestamp = valid_payload.timestamp + 1;
-        let random = Hash256::zero();
+        let prev_randao = Hash256::zero();
         let finalized_block_hash = ExecutionBlockHash::zero();
         let proposer_index = 0;
         let second_payload = self
@@ -236,7 +236,7 @@ impl<E: GenericExecutionEngine> TestRig<E> {
             .get_payload::<MainnetEthSpec>(
                 parent_hash,
                 timestamp,
-                random,
+                prev_randao,
                 finalized_block_hash,
                 proposer_index,
             )
@@ -266,7 +266,7 @@ impl<E: GenericExecutionEngine> TestRig<E> {
         let finalized_block_hash = ExecutionBlockHash::zero();
         let payload_attributes = Some(PayloadAttributes {
             timestamp: second_payload.timestamp + 1,
-            random: Hash256::zero(),
+            prev_randao: Hash256::zero(),
             suggested_fee_recipient: Address::zero(),
         });
         let status = self