clients/erigon: change p2p version priorities to fix several Erigon devp2p tests

[cryptoscruffy]

Use 68 version as default. This fixes 5 failed devp2p tests for Erigon

[holiman]

Hm... so is the end-user supposed to know this, and set those flags explicitly?

[fjl]

Yeah, I think the same thing, why are these flags not default? cc. @yperbasis

[cryptoscruffy]

As far as I know end users use --p2p.protocol setting often enough, but I don't sure it is a correct way to resolve the issue. I've tried to copy accepted #766 PR, reject my PR please if you think it is wrong approach.

[fjl]

@cryptoscruffy Your PR is totally fine, we are just wondering in general why erigon requires this flag to activate the protocol versions. We are not expecting an answer from you about this.

[cryptoscruffy]

Looks like 67 version is default now just because it was not updated in Erigon, so the correct way to fix this issue is to update Erigon's defaults. I'll try to make PR to Erigon's repo instead of this one.