Using `matrix_synapse_*` variables within the `matrix-grafana` role
is not a good practice.
We now have a `matrix_grafana_default_home_dashboard_path` variable
with a good universal default value and we override it via
`group_vars/matrix_servers` based on enabled components, etc.
This is a better fix for https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/2133
Also includes the dashboards for Synapse and for Node Exporter.
Again has only been tested on debian amd64 so far, but the grafana docker image is available for arm64 and arm32. Nice.