Update Nethermind in exec integration tests

[michaelsproul]

Description

While working on #3982 I tried upgrading Nethermind to a more recent version as well. I ran into an issue where the kiln config is missing from the new version of Nethermind, so this flag is invalid:

lighthouse/testing/execution_engine_integration/src/nethermind.rs

Lines 97 to 98 in 38514c0

	.arg("--config")
	.arg("kiln")

We're currently pinned on Nethermind v1.14.6 where kiln is still a valid concept:

lighthouse/testing/execution_engine_integration/src/nethermind.rs

Line 14 in 38514c0

const NETHERMIND_BRANCH: &str = "release/1.14.6";

I tested the latest stable at time of writing v1.16.1 and it doesn't support Kiln any longer.

Steps to resolve

I think we need a new genesis.json that's not based on Kiln, here:

lighthouse/testing/execution_engine_integration/src/genesis_json.rs

Lines 49 to 50 in 38514c0

	/// Modified kiln config
	pub fn nethermind_genesis_json() -> Value {

We could likely use one of the genesis generator tools. Related: #3807.

[michaelsproul]

Solved by @ethDreamer in #4028 🎉