This commit is somewhat of a bandaid fix for a bug that was revealed in #38029
and then #38151. #38151 can cause problems in certain cases when an app has
a 3-tier config, with replicas, because it reorders the configuration
and changes the implict default connection that gets picked up.

If an app calls `establish_connection` with no arguments or doesn't call
`connects_to` in `ApplicationRecord` AND uses parallel testing
databases, the application may pick up the wrong configuration.

This is because when the code in #38151 loops through the configurations
it will update the non-replica configurations and then put them at the
end of the list. If you don't specify which connection you want, Rails
will pick up the _first_ connection for that environment. So given the
following configuration:

```
test:
  primary:
    database: my_db
  replica:
    database: my_db
    replica: true
```

The database configurations will get reordered to be `replica`,
`primary` and when Rails calls `establish_connection` with no arguments
it will pick up `replica` because it's first in the list.

Looking at this bug it shows that calling `establish_connection` with no
arguments (which will pick up the default env + first configuration in
the list) OR when `establish_connection` is called with an environment
like `:test` it will also pick up that env's first configuration. This
can have surprising behavior in a couple cases:

1) In the parallel testing changes we saw users getting the wrong db
configuration and hitting an `ActiveRecord::ReadOnlyError`
2) Writing a configuration that puts `replica` before `primary`, also
resulting in a `ActiveRecord::ReadOnlyError`

The real fix for this issue is to deprecate calling
`establish_connection` with an env or nothing and require an explcit
configuration (like `primary`). This would also involve blessing
`:primary` as the default connection Rails looks for on boot. In
addition, this would require us deprecating connection specification
name "primary" in favor of the class name always since that will get
mega-confusing (seriously, it's already mega-confusing).

We'll work on fixing these underlying issues, but wanted to get a fix
out that restores previous behavior.
Co-authored-by: NJohn Crepezzi <john.crepezzi@gmail.com>