fix(genesis): Set Isthmus timestamp to 0 to activate Prague hardfork

The Isthmus maps to Prague hardfork, previously activated by the prague timestamp only

Author: RomanHodulak

etc/odyssey-genesis.json

@@ -19,6 +19,7 @@
     "regolithTime": 0,
     "ecotoneTime": 0,
     "pragueTime": 0,
+    "isthmusTime": 0,
     "terminalTotalDifficulty": 0,
     "terminalTotalDifficultyPassed": true,
     "optimism": {