a11y : Color contrast issues on Alert Messages and Toast components

[jblagadec]

This ticket is similar to #334 .

On Alert messages :

To meet the contrast criteria (at least 3:1 for meaningful icons):

• on the success state, we should replace Green Success 600 by Success 700
• on the Warning state, we should replace Orange Warning 600 by Warning 700

On toast componants :

• on the success state, we should replace Green Success 500 by Success 700
• on the Warning state, we should replace Orange Warning 500 by Warning 700