Fixing SharedModel.connection_db_config when connection is changed

What does this MR do and why?

Simple solution to addressing: #386242 (closed)

Other possible solution as well

I was thinking of overriding connection_db_config instead of connection_pool, but then I saw this implementation of connection_db_config, and I thought we can change it on the source connection_pool

Other thoughts

I was thinking of solving it on the LoadBalancer::Setup somewhere here, so that it's also solved for any other classes. But I didn't want to complicate the solution, because we still have this bigger issue and this one

How to set up and validate locally

LooseForeignKeys::DeletedRecord.using_connection(Ci::ApplicationRecord.connection) do
  LooseForeignKeys::DeletedRecord.connection_db_config.name
end

Should return ci on this branch, which is what we expected. But it returns main on master

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Related to #386242 (closed)