Migrate to new library

[DifferentialOrange]

grafana/grafonnet-lib is deprecated now in favor of grafana/grafonnet. The main advantage is that new library is automatically generated from Grafana code itself, so any new updates should be fetched automatically or self-automatically.

Since it doesn't seem that old panels would be dropped soon, we may not hurry too much. But it's better not to ignore it too much too.

We may need to rework generate grid and ensure that 6.x graph panels would look fine if we replace it with 8.x graph panels.