Make aria-label mandatory for buttons

[ChristophWurst]

We've logged a warning for a while

You need to fill either the text or the ariaLabel props in the button component

I think it would be fair to start enforcing this prop in a future major version.

Ref https://github.com/nextcloud/nextcloud-vue/commit/d441b9dfb47aebae17bae4c21493057e4a7c862c @marcoambrosini

[susnux]

From the discussion here it seems we should set the aria label always to the visual text and use a different attribute (title? aria-description?) for more context.