Remove libp2p multiaddress

[jxs]

After a user reported on Discord reported having problems starting Lighthouse with discovery disabled and providing --boot-nodes
I noticed that when a bootnode is provided as ENR lighthouse connects to it directly using libp2p, when a bootnode is a multiaddress it needs to be a UDP multiaddress so that Lighthouse uses Discovery to fetch that node's ENR.
But then later when bootstrapping libp2p, Lighthouse tries to dial those multiaddresses directly from libp2p if they are TCP, thing is, they won't be, as that was not allowed when the config was parsed.
Lighthouse also supports libp2p-nodes config flag that dials the addresses provided in the same way as when providing a valid ENR.

This PR basically removes the --libp2p-nodes flag and uniforms on boot-nodes flag allowing users to pass TCP multiaddresses with the --boot-nodes flag

[ackintosh]

Thanks @jxs for the cleanup! ✨ Looks good to me.

One small thought: how about having a deprecation period for the --libp2p-nodes flag to preserve CLI compatibility? We usually do this for CLI changes -- keeping the old flag as an alias, emitting a warning when used ( e.g. the --logfile deprecation in #7723).

[pawanjay176]

This looks clean. I had no idea we accepted multiaddrs in boot-nodes as well.
Agree with @ackintosh about the deprecation instead of delete. I think some existing infra might depend on the flag.