Modal titles no longer use a heading font size

Since XWIKI-15726, modal titles are divs styled with a fixed font-size so that they don't break the headings hierarchy. That size was picked to match the H4 size of the time, around 20px.

XWIKI-21333 later changed the heading sizes and made them fluid with the window width, but the modal title size stayed fixed. Modal titles now render at about 20px while H4 is about 16-17px, so they don't match any heading size anymore.

Possible ways to make them consistent again:

• use the H3 size, which reaches about 20px on a wide window, so the change would be barely visible
• use the H4 size, which would make every modal title visibly smaller
• keep a dedicated size and document it as a design decision

This came up in the review of https://github.com/xwiki/xwiki-platform/pull/6598#discussion_r4185225108. A forum proposal will settle which option to choose.