As multiple databases have evolved it's becoming more and more
confusing that we have a `connection_specification_name` that defaults
to "primary" and a `spec_name` on the database objects that defaults to
"primary" (my bad).

Even more confusing is that we use the class name for all
non-ActiveRecord::Base abstract classes that establish connections. For
example connections established on `class MyOtherDatabaseModel <
ApplicationRecord` will use `"MyOtherDatabaseModel"` as it's connection
specification name while `ActiveRecord::Base` uses `"primary"`.

This PR deprecates the use of the name `"primary"` as the
`connection_specification_name` for `ActiveRecord::Base` in favor of
using `"ActiveRecord::Base"`.

In this PR the following is true:

* If `handler.establish_connection(:primary)` is called, `"primary"`
will not throw a deprecation warning and can still be used for the
`connection_specification_name`. This also fixes a bug where using this
method to establish a connection could accidentally overwrite the actual
`ActiveRecord::Base` connection IF that connection was not using a
configuration named `:primary`.
* Calling `handler.retrieve_connection "primary"` when
`handler.establish_connection :primary` has never been called will
return the connection for `ActiveRecord::Base` and throw a deprecation
warning.
* Calling `handler.remove_connection "primary"` when
`handler.establish_connection :primary` has never been called will
remove the connection for `ActiveRecord::Base` and throw a deprecation
warning.

See #38179 for details on more motivations for this change.

Co-authored-by: John Crepezzi <john.crepezzi@gmail.com>