Wizard step icons are not in line with the button style

Recently, the style of buttons have been updated to a more flat design (see https://github.com/xwiki/xwiki-platform/commit/9be2ea24c07e557de1583b63c1130fc9bd0a8949 ). However, wizard step 'numbers' did not change acoording to those changes.

1. They should fit the new style.
2. Their style should use a mixin or a btn class to avoid a similar issue on long term

On this screenshot, we can see that the shadow on the `.steps .number` elements make them stand out in an unfavorable way.