Changes to rustic config

[simonsan]

I think it might be also confusing to have [[backup.sources]] multiple times and then to have a field called source which takes multiple values again.

The field should probably be renamed to sources and the table name to [[backup.source-groups]] or something. So it's clearer. Then sources takes an array of paths: ["/path/1/2", "path/3/4"].

Originally posted by @simonsan in https://github.com/rustic-rs/rustic/issues/1094#issuecomment-1984376160

• since #1124 source is now an array