Nimbus VC with Lighthouse BN never exits doppelganger

[jshufro]

Describe the bug
A Nimbus VC with at least one active validator, when connected to a Lighthouse BN, will never exit doppelganger detection.

Additional context
Observed when we temporarily redirected rescue node dns for maintenance, and initially dismissed as "probably something we did", but reaffirmed by @wrobinson in the rocket pool discord when using nimbus with a lighthouse fallback

[arnetheduck]

Right - last we checked, lighthouse had not yet implemented the standard liveness request needed for doppelganger detection to work.

If you have logs, we can take a look at them and see if it's possible to improve them to point this out, but without a beacon node that implements the prerequisite API, the VC can never "know" that it's safe to move out of DG.

[jshufro]

They have implemented it here: sigp/lighthouse#4343

I mistakenly thought this was already working and regressed, though.

I will try to get my hands on some logs