fix: add-isthmus-on-arenaz-testnet (#1104)

Author: HiteshMittal07

superchain/configs/sepolia/arena-z.toml

@@ -18,6 +18,7 @@ max_sequencer_drift = 600
   fjord_time = 0 # Thu 1 Jan 1970 00:00:00 UTC
   granite_time = 0 # Thu 1 Jan 1970 00:00:00 UTC
   holocene_time = 0 # Thu 1 Jan 1970 00:00:00 UTC
+  isthmus_time = 1747839600 # Wed 21 May 2025 15:00:00 UTC
 
 [optimism]
   eip1559_elasticity = 6