Make unknown gameserver log debug

[XAMPPRocky]

This log isn't that useful because it mostly just pops up as an eventual consistency issue (e.g. we often receive multiple delete events for a server). So it's just noise.

[quilkin-bot]

Build Succeeded 🥳

Build Id: a2aa5e07-b803-4ae4-af53-0357f35ebd17

The following development images have been built, and will exist for the next 30 days:

• us-docker.pkg.dev/quilkin/ci/quilkin:0.9.0-dev-65bb159

To build this version:

git fetch git@github.com:googleforgames/quilkin.git pull/923/head:pr_923 && git checkout pr_923
cargo build