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.ts

@@ -108,7 +108,7 @@ export class CodeComponent implements OnChanges {
 
     if (!this.code) {
       const src = this.path ? this.path + (this.region ? '#' + this.region : '') : '';
-      const srcMsg = src ? ` for<br>${src}` : '.';
+      const srcMsg = src ? ` for\n${src}` : '.';
       this.setCodeHtml(`<p class="code-missing">The code sample is missing${srcMsg}</p>`);
       return;
     }
@@ -129,8 +129,8 @@ export class CodeComponent implements OnChanges {
   }
 
   doCopy() {
-    // We take the innerText because we don't want it to be HTML encoded
-    const code = this.codeContainer.nativeElement.innerText;
+    // We take the textContent because we don't want it to be HTML encoded
+    const code = this.codeContainer.nativeElement.textContent.trim();
     if (this.copier.copyText(code)) {
       this.logger.log('Copied code to clipboard:', code);
       // success snackbar alert