Change all Grafana dashboards to fixed UID data sources
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Apply on top of !357 (merged).
This sets fixed "unique" IDs for our two data sources Prometheus and Loki - to be the same over all deployments - and changes all dashboard JSON definitions to make use of that by changing all datasource references to use the "U"IDs.
Setting "unique" IDs to be the same over different deployments does not feel quite right, but seems to be the recommended approach by Grafana homies.