Update to spec v1.1.8 (#2893)

## Proposed Changes

Change the canonical fork name for the merge to Bellatrix. Keep other merge naming the same to avoid churn.

I've also fixed and enabled the `fork` and `transition` tests for Bellatrix, and the v1.1.7 fork choice tests.

Additionally, the `BellatrixPreset` has been added with tests. It gets served via the `/config/spec` API endpoint along with the other presets.

common/eth2_network_config/built_in_network_configs/mainnet/config.yaml

@@ -33,8 +33,8 @@ GENESIS_DELAY: 604800
 ALTAIR_FORK_VERSION: 0x01000000
 ALTAIR_FORK_EPOCH: 74240
 # Merge
-MERGE_FORK_VERSION: 0x02000000
-MERGE_FORK_EPOCH: 18446744073709551615
+BELLATRIX_FORK_VERSION: 0x02000000
+BELLATRIX_FORK_EPOCH: 18446744073709551615
 # Sharding
 SHARDING_FORK_VERSION: 0x03000000
 SHARDING_FORK_EPOCH: 18446744073709551615

common/eth2_network_config/built_in_network_configs/prater/config.yaml

@@ -33,8 +33,8 @@ GENESIS_DELAY: 1919188
 ALTAIR_FORK_VERSION: 0x01001020
 ALTAIR_FORK_EPOCH: 36660
 # Merge
-MERGE_FORK_VERSION: 0x02001020
-MERGE_FORK_EPOCH: 18446744073709551615
+BELLATRIX_FORK_VERSION: 0x02001020
+BELLATRIX_FORK_EPOCH: 18446744073709551615
 # Sharding
 SHARDING_FORK_VERSION: 0x03001020
 SHARDING_FORK_EPOCH: 18446744073709551615

common/eth2_network_config/built_in_network_configs/pyrmont/config.yaml

@@ -33,8 +33,8 @@ GENESIS_DELAY: 432000
 ALTAIR_FORK_VERSION: 0x01002009
 ALTAIR_FORK_EPOCH: 61650
 # Merge
-MERGE_FORK_VERSION: 0x02002009
-MERGE_FORK_EPOCH: 18446744073709551615
+BELLATRIX_FORK_VERSION: 0x02002009
+BELLATRIX_FORK_EPOCH: 18446744073709551615
 # Sharding
 SHARDING_FORK_VERSION: 0x03002009
 SHARDING_FORK_EPOCH: 18446744073709551615