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/lib.rs

@@ -392,7 +392,7 @@ impl ExecutionLayer {
         &self,
         parent_hash: ExecutionBlockHash,
         timestamp: u64,
-        random: Hash256,
+        prev_randao: Hash256,
         finalized_block_hash: ExecutionBlockHash,
         proposer_index: u64,
     ) -> Result<ExecutionPayload<T>, Error> {
@@ -402,14 +402,14 @@ impl ExecutionLayer {
             self.log(),
             "Issuing engine_getPayload";
             "suggested_fee_recipient" => ?suggested_fee_recipient,
-            "random" => ?random,
+            "prev_randao" => ?prev_randao,
             "timestamp" => timestamp,
             "parent_hash" => ?parent_hash,
         );
         self.engines()
             .first_success(|engine| async move {
                 let payload_id = if let Some(id) = engine
-                    .get_payload_id(parent_hash, timestamp, random, suggested_fee_recipient)
+                    .get_payload_id(parent_hash, timestamp, prev_randao, suggested_fee_recipient)
                     .await
                 {
                     // The payload id has been cached for this engine.
@@ -428,7 +428,7 @@ impl ExecutionLayer {
                     };
                     let payload_attributes = PayloadAttributes {
                         timestamp,
-                        random,
+                        prev_randao,
                         suggested_fee_recipient,
                     };