Bottom padding too large in box/message macros

Steps to reproduce:

Open the Sandbox page.

Expected result:

The first message looks like this:

Actual result:

The first message looks like this:

The padding at the bottom is much larger than at the top, which looks weird.

This is because the CSS rule to remove the padding at https://github.com/xwiki/xwiki-platform/blob/e5af673a29e4666cfd9fb0a3e576cd52131786c8/xwiki-platform-core/xwiki-platform-flamingo/xwiki-platform-flamingo-skin/xwiki-platform-flamingo-skin-resources/src/main/resources/flamingo/less/messages.less#L65-L70 isn't applied anymore due to the additional div container introduced in XRENDERING-768.