Update default RPC provider in sncast

[franciszekjob]

Current State

In the end of October, blast provider will be shut down. We should update our default provider in sncast, e.g. candidate is equlibrium (from pathfinder).
url: https://rpc.pathfinder.equilibrium.co/testnet-sepolia/rpc/v0_9

Objective

Update default RPC provider in sncast

Additional Context

No response