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.component.spec.ts

@@ -116,7 +116,7 @@ describe('CodeComponent', () => {
       hostComponent.linenums = false;
       hostComponent.code = '  abc\n   let x = text.split(\'\\n\');\n  ghi\n\n  jkl\n';
       fixture.detectChanges();
-      const codeContent = codeComponentDe.nativeElement.querySelector('code').innerText;
+      const codeContent = codeComponentDe.nativeElement.querySelector('code').textContent;
       expect(codeContent).toEqual('abc\n let x = text.split(\'\\n\');\nghi\n\njkl');
     });
 
@@ -124,7 +124,7 @@ describe('CodeComponent', () => {
       hostComponent.linenums = false;
       hostComponent.code = '\n\n\n' + smallMultiLineCode + '\n\n\n';
       fixture.detectChanges();
-      const codeContent = codeComponentDe.nativeElement.querySelector('code').innerText;
+      const codeContent = codeComponentDe.nativeElement.querySelector('code').textContent;
       expect(codeContent).toEqual(codeContent.trim());
     });
 
@@ -141,7 +141,7 @@ describe('CodeComponent', () => {
 
     function getErrorMessage() {
       const missing: HTMLElement = codeComponentDe.nativeElement.querySelector('.code-missing');
-      return missing ? missing.innerText : null;
+      return missing ? missing.textContent : null;
     }
 
     it('should not display "code-missing" class when there is some code', () => {