An apologia, playwright Alexi Kaye Campbell reminds us in his play so titled, is a defense, not an apology. With that in mind, would any play need an apologia beyond the words “Stockard Channing?” You wouldn’t think so, and you’d be close to right, but Apologia, directed by Daniel Aukin and opening tonight at the Roundabout Theatre Company’s Off Broadway Laura Pels venue, could use some bolstering nonetheless. Channing reprises a role she created in London, and we can…