docs(repo): update warnings in README

We should update this, since the mainnet has been forked into Shasta, the new branch to check is `taiko-alethia-protocol-v3.0.0`: > The main branch is under active development for the next Shasta fork. For the latest version of the Taiko Alethia protocol contracts (Pacaya fork), please use the [taiko-alethia-protocol-v2.3.1](https://github.com/taikoxyz/taiko-mono/tree/taiko-alethia-protocol-v2.3.1) branch. The release process involves security measures that the main branch does not guarantee.