Update Sepolia Testnet Block Time to 12 Seconds

[gacevicljubisa]

Context

The block time for the testnet (Sepolia) should be updated to 12 seconds.

Summary

There is a hardcoded value used to determine the current average block time based on the network ID. Currently, this value does not reflect the correct block time for Sepolia.

Expected behavior

The block time for the Sepolia chain should be 12 seconds.

Actual behavior

The current block time for the Sepolia chain is set to 15 seconds.

Steps to reproduce

Refer to the following line in the Bee repository:
cmd/bee/cmd/start.go#L465

Possible solution

Update the hardcoded value to 12 seconds for the testnet.