Remove renderTemplate, disconnectOutlet, render

[mixonic]

Removing these features also effectively removes support for named outlets, however I did not update the docs or code to prevent a name from being passed to {{outlet 'foo'}} as it was not explicitly deprecated.

Per #19617

[mixonic]

The comment here around {{outlet 'foo'}} being dropped as API was discussed on July 23rd with Ember Framework Core. We agreed that {{outlet 'foo'}} should become an assertion in dev builds. In a production build, 'foo' is an argument passed where {{outlet}} would expect none, and so it would be silently ignored. Asserting in dev will prevent users who expect an argument here from stumbling into an unexpected behavior.

edit: Implemented at #19691