There was no need to add `synapse` to the list manually
and then add all other additional databases.
When the `synapse` database was the main database, this made sense.
Since a long time ago already, ALL databases are "additional" databases,
so the `synapse` database is part of that list.
We could additional add the main (`matrix`) database to this list,
but there's probably no point in backing that one up.
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