Bump version to discv5.1

[djrtwo]

trafficstars

As discussed on last weeks networking call, we decided to bump discovery up to version 5.1.

What is the state of language implementations? -- Please reply with not_started, in_progress, complete and provide any relevant links to issues/PRs

Language	Status	Links
Java	in_progress	https://github.com/PegaSysEng/discovery/pull/72
Rust	complete (need interop testing)	https://github.com/sigp/lighthouse/issues/1605
Nim	in_progress	https://github.com/status-im/nim-beacon-chain/issues/1649
Go	complete (reference impl)	https://github.com/ethereum/devp2p/pull/157

Note, this is incompatible with with the current version deployed on Medalla. To be determined if/how we upgrade Medalla, but we intend to use this on mainnet.

[ajsutton]

Teku: not_started https://github.com/PegaSysEng/teku/issues/2797

[AgeManning]

Lighthouse has started. This issue will be marked complete once it's integrated into Lighthouse: https://github.com/sigp/lighthouse/issues/1605

In regards to updating. If we update one of the current bootnodes to 5.1, then 5.1 nodes can bootstrap. And as Lighthouse clients update to 5.1, when they restart they will re-inject past known ENR's into the 5.1 routing table, essentially joining the DHT's, I imagine.

[kdeme]

Nimbus has just started, so in_progress. Tracking issue: https://github.com/status-im/nim-beacon-chain/issues/1649

[Nashatyrev]

Teku has also started: https://github.com/PegaSysEng/discovery/pull/72

[AgeManning]

We have finished our implementation, pending some interop testing.