fix: update RPC and Block Explorer URLs

[0xemrekaya]

This PR updates the RPC and Block Explorer URLs for Scroll Network. These changes reflect the recent configuration changes in Scroll Network.

Changes:

• The RPC URL has been updated from the Alpha testnet to the Sepolia testnet.
• The Mainnet RPC URL has been added.
• The Testnet Block Explorer URL has been updated from "https://blockscout.scroll.io" to "https://sepolia.scrollscan.com".
• The Mainnet Block Explorer URL, "https://scrollscan.com/", has been added.
• README.md file has been updated

These changes are necessary to keep up with the recent configuration updates in Scroll Network. They ensure that the application interacts correctly with the network.

Tests: The changes have been manually tested to confirm compatibility with the new configuration of Scroll Network. The tests involved making requests to the updated URLs and verifying the responses. The application's functionality was also tested to ensure it operates correctly with the new URLs.