contracts-bedrock: update sepolia deploy config

See https://github.com/ethereum-optimism/optimism/pull/8515 for
explanation. Also related to https://github.com/ethereum-optimism/optimism/pull/8519.

Value fetched from:
https://sepolia.etherscan.io/address/0xD5bAc3152ffC25318F848B3DD5dA6C85171BaEEe#readProxyContract

Address sourced from:
https://github.com/ethereum-optimism/superchain-registry/blob/fb3433d89a72df5809255c065b4783fa393608d8/superchain/extra/addresses/sepolia/pgn.json#L6C27-L6C69

packages/contracts-bedrock/deploy-config/pgn-sepolia.json

@@ -22,7 +22,7 @@
   "l2GenesisRegolithTimeOffset": "0x0",
   "superchainConfigGuardian": "0x45eFFbD799Ab49122eeEAB75B78D9C56A187F9A7",
   "l2OutputOracleSubmissionInterval": 180,
-  "l2OutputOracleStartingTimestamp": -1,
+  "l2OutputOracleStartingTimestamp": 1685727972,
   "l2OutputOracleProposer": "0xD457799C5ba870D609f21048c67a9b11aC611BF0",
   "l2GenesisBlockGasLimit": "0x1c9c380",
   "fundDevAccounts": false,