Use standard values for font weight CSS variables

The current CSS variables in XWiki for bolder text are the following:

--font-weight-semibold: 700;

--font-weight-bold: 900;

However, the standard definition for semibold styles in font families is to have it at 600. While 700 is for bold properly and 900 is called a black style (meaning, even bolder).

This is affecting design proposals and implementations that use the semibold styles. 

For customizations that use fonts that don't have any semibold style in their family, the next value is used by default, so we don't have a risk of "bolder" text appearing as "regular".

Documentation explaining all values and browser behavior.

https://developer.mozilla.org/en-US/docs/Web/CSS/Reference/At-rules/@font-face/font-weight