Rework with_disabled_database_connections to work with Rails 7.0

What does this MR do and why?

Note: the original motivation is from #359913 (closed)

Note 2: Due to changes, the method is now renamed to raise_if_new_database_connection

In Rails 7.0, attribute definitions in models now run at class definition time, and calls model.connection_db_config. This means swapping the connection_handler now breaks because there's no valid connection_pool within with_disabled_database_connections block. See https://github.com/rails/rails/pull/41166, and https://github.com/rails/rails/pull/41302

On investigation, .connection_pool, and .connection_db_config does not create a database connection (model.connection does).

In order to have this keep working in Rails 7.0, we switch to counting the number of connections before, and after instead.

Also to prevent re-use of connections, we also disconnect all active connections first (already implemented prior, see clear_active_connections_again)

Related issue: #389879 (closed)

Screenshots or screen recordings

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How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Add Project.first, or Ci::Pipeline.first inside the with_disabled_database_connections block in config/routes.rb
2. You should see an error: Database connection should not be called during initializers

Also make sure that Rails boots in Rails 7 :

1. Check out !90907 (merged)
2. Cherry pick commits from this branch
3. Reverse the temporary change to lib/initializer_connections.rb (!90907 (diffs))
4. Test that bundle exec rails console still works

MR acceptance checklist

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