Page buttons background follows @breadcrumb-bg value

This style relies on the mixin `.flat-buttons` which is used in multiple places. This mixin was introduced in https://github.com/xwiki/xwiki-platform/commit/c974f9ca8d3036b5649e12f3307543976214064b as a way to factorize code. From its name and the description, it was used mostly to get nice looking default style buttons. My guess is that the gradient we used to have on them made them difficult to use anywhere, and this was a correct alternative that was used when in need of a nice looking default style.

This mixin is specific to XWiki and is not mentionned anywhere in the documentation (see https://www.xwiki.org/xwiki/bin/view/Documentation/DevGuide/FrontendResources/).

In my opinion we can consider that it was not part of a front end API (there never was one, but some styles are more critical for customizations).

Today and since we moved to flat buttons everywhere (https://github.com/xwiki/xwiki-platform/pull/2316 and XWIKI-21253 ), this mixin is completely useless. It only brings in inconsistency in the style and somewhat duplicated styles.

That's why I think the solution to this ticket should aim at removing the flat-buttons mixin altogether.