fix(aio): switch from innerText to textContent to support older browsers

`innerText` is not supported in Firefox prior to v45. In most cases (at least
the ones we are interested in), `innerText` and `textContent` work equally well,
but `textContent` is more performant (as it doesn't require a reflow).

From [MDN][1] on the differences of `innerText` vs `textContent`:

> - [...]
> - `innerText` is aware of style and will not return the text of hidden
>   elements, whereas `textContent` will.
> - As `innerText` is aware of CSS styling, it will trigger a reflow, whereas
>   `textContent` will not.
> - [...]

[1]: https://developer.mozilla.org/en-US/docs/Web/API/Node/textContent#Differences_from_innerText

Fixes #17585

aio/src/app/embedded/code/code-example.component.spec.ts

@@ -62,7 +62,7 @@ describe('CodeExampleComponent', () => {
     TestBed.overrideComponent(HostComponent, {
       set: {template: '<code-example title="Great Example"></code-example>'}});
     createComponent(oneLineCode);
-    const actual = codeExampleDe.query(By.css('header')).nativeElement.innerText;
+    const actual = codeExampleDe.query(By.css('header')).nativeElement.textContent;
     expect(actual).toBe('Great Example');
   });