Feature: Downgrade `Snapshot extension registration failed` to `WARN`

[yorickdowne]

Rationale

Given that ERROR[04-06|13:38:24.463] Snapshot extension registration failed peer=2005b987 err="peer connected on snap without compatible eth support" is a) common and b) not at all in the control of the operator, downgrading this message to WARN would simplify alerting.

Ideally, ERROR and CRIT logs are issued for things that are indeed an issue and under the control of the operator to do something about.

[holiman]

Ok, done: #24475 :)