Migrate Arbitrum Network from Goerli to Sepolia

[avichalp]

Summary

This pull request includes changes to migrate the Arbitrum network from Goerli to Sepolia.

Key changes include:

Update ChainIDs: The ChainID for Arbitrum has been updated from ArbitrumGoerli to ArbitrumSepolia in the chains.go file. The corresponding ChainID number has also been updated.

Update Configurations: The .env_validator.example and config.json files have been updated to reflect the change in the Arbitrum network. The API key and endpoint for Arbitrum have been updated from Goerli to Sepolia.

Update Endpoints: The endpoint URLs for Arbitrum in the chains.go file have been updated to point to the Sepolia network.

Update Contract Address: The contract address for the Arbitrum network in the chains.go file has been updated.

These changes ensure that the application is now interacting with the Sepolia network for Arbitrum instead of Goerli.

• Are changes backward compatible with existing SDKs, or is there a plan to manage it correctly?
• Are changes covered by existing tests, or were new tests included?
• Are code changes optimized for future code readers, commenting on problematic areas to understand (if any)?
• Future-self question: Did you avoid unjustified/unnecessary complexity to achieve the goal?
• Add required ENV vars

NOTE: Doesn't add the healthbot config since we have stopped using healthbot on optimism and arbitrum

[sanderpick]

lgtm!