drop polygon mumbai

[avichalp]

• This PR removes support for polygon mumbai as it will be deprecated on April 13th 2024.
• Until we have Polygon Amoy support ready, this PR changes the default network to Ethereum Sepolia

Polygon has informed us that they will deprecate Mumbai on April 13. We are planning on keeping our Mumbai nodes for Polygon running until this date. However, in the past we have seen extreme instability in networks leading up to deprecation. While we will do our best to keep it running, we cannot guarantee that this network will be usable until April 13. It is in your best interest to migrate to Amoy immediately. If you try to send requests to these nodes after Alchemy support ends, your requests will fail with a DNS resolution error.

[brunocalza]

Looks good! Although, I think the ideal transition would be to first offer Amoy, so users have a chance to migrate first, then drop Mumbai. But since we still don't have an Amoy contract, I don't think we have an option.