gnome-shell: Reduce icon grid spacing

Based on upstream commit:
https://gitlab.gnome.org/GNOME/gnome-shell/-/commit/5dba92815483ad38dab0588104b69e8412caf48d

Author: jnsh

common/gnome-shell/3.36/sass/_common.scss

@@ -1899,7 +1899,7 @@ StScrollBar {
 // App Vault/Grid
 //
 .icon-grid {
-  spacing: 30px;
+  spacing: 20px;
   -shell-grid-horizontal-item-size: 136px;
   -shell-grid-vertical-item-size: 136px;