Adjust icon in umb-checkbox and ensure icon is centered

(cherry picked from commit c49c536)

Author: bjarnef

src/Umbraco.Web.UI.Client/src/less/components/umb-form-check.less

@@ -97,7 +97,8 @@
 
     .icon,
     .umb-icon {
-        font-size: 1.2rem;
+        font-size: 0.9rem;
+        line-height: 1;
     }
 
     &__state {
@@ -110,6 +111,8 @@
 
     &__check {
         display: flex;
+        align-items: center;
+        justify-content: center;
         position: relative;
         background: @white;
         border: 1px solid @inputBorder;