Use bootstrap_nodes.yaml instead of boot_enr.yaml in testnet directories

[michaelsproul]

Currently devnet and testnet tools tend to use two files for bootnodes:

• bootstrap_nodes.txt
• bootstrap_nodes.yaml

Example for pectra-devnet-6 here.

Neither are supported by Lighthouse. We expect a file in the --testnet-dir called boot_enr.yaml, which as far as I know is a name not used by any other client.

I think it would be relatively straight-forward for us to switch to the bootstrap_nodes.yaml filename. It would be a breaking change, but one affecting a minimal number of users. We could update all our testnet tooling and bundled network configs very easily.