[37404] Remove fixed height of ng-select container (#9307)

Instead, set min-height to allow minimum and set the placeholder length
to 2rem to match the height when its empty.

This still allows the height to expand for multiple options

https://community.openproject.org/wp/37404

frontend/src/global_styles/vendor/_ngselect.sass

@@ -5,4 +5,7 @@
   border-radius: 2px
 
 .ng-select .ng-value-container
-  height: 2rem
+  min-height: 2rem
+
+.ng-placeholder
+  line-height: 2rem