Set select height to 40px

The `min-height` property for selects is ignored in Safari, making the select only 30px high. Setting a fixed height is ok as the select will never wrap onto 2 lines.

packages/components/select/_select.scss

@@ -7,7 +7,7 @@
 
   border: $nhsuk-border-width-form-element solid $nhsuk-form-border-color;
   box-sizing: border-box;
-  min-height: 40px;
+  height: 40px;
   max-width: 100%;
   padding: nhsuk-spacing(1);