Use `attrs().mergeWith()` in Twig code examples

[m-vo]

We need to adjust the Twig code examples regarding the set and merge pattern to reflect the changes made in contao/contao#6956 (comment).

Basically attrs(foo).<operations> → attrs().<operations>.mergeWith(foo) .

[fritzmg]

I started working on this but now I am not sure anymore that this is what we want. Or at least not everywhere. If you create a new template for your new content element for example, then yes. But when you overwrite a template in your App, then no? (Or it simply does not matter then?)

[m-vo]

Technically, it would only matter in your app if you are using themes. Because then, a theme template can also adjust your app template… And if you mean @Contao_App (contao/templates) by app, then also every backend admin that can adjust templates could adjust the "app" templates.