fix: update welcome-height to 80vh (#21)

Author: shenxianpeng

themes/devops-maturity/static/css/scss/abstracts/_variables.scss

@@ -22,7 +22,7 @@ $header-height: 56px;
 $footer-logo-size: 24px;
 
 $welcome-min-height: 450px;
-$welcome-height: 30vh;
+$welcome-height: 80vh;
 $welcome-action-color: $color-neutral-light;
 
 $main-background: $color-neutral-light;