wastewater dashboards: allow automatic update of default variant in variant explorer

[fhennig]

low prio

We have hard-coded variants that are currently the most prevalent for Covid, RSV-A, and RSV-B, to be the default variant in the variant explorer. Eventually it would be nice if this was somehow automatically pulled from somewhere.

[fhennig]

Maybe we could even have a GitHub action that somehow makes some LAPIS queries for each organism and finds the most prevalent lineage that way, and then automatically creates PR(s) to update the config 😅