.maintain: Replace sentry-node with local-docker-test-network (#7943)

Sentry nodes are deprecated. Thus there is no need for
`.maintain/sentry-node` to spin up a sentry node test environment.
Instead this commit rewrites the setup to contain two full-connected
validators and one light client.

With the steps below one can now spin up a local test network with
two validators, one light-client, Prometheus and Grafana.

- cargo build --release
- sudo docker-compose -f .maintain/local-docker-test-network/docker-compose.yml up