Only morph the turbo-frame contents, not the frame itself

I had removed this in #d1935bd15c85e0a8776afccb90393fd378aea2d2 but we do
need innerHTML so that the outer frame don't get touched. The problem we had
is that we were nesting turbo-frames, so using .children to only address the
contents instead.

src/core/drive/morph_renderer.js

@@ -58,7 +58,7 @@ export class MorphRenderer extends Renderer {
       target: currentElement,
       detail: { currentElement, newElement }
     })
-    this.#morphElements(currentElement, newElement)
+    this.#morphElements(currentElement, newElement.children, "innerHTML")
   }
 
   #shouldRemoveElement = (node) => {