Use selection widget for weekday on currency cron form

Closes #12292

modules/currency/view/cron_form.xml

@@ -8,7 +8,7 @@ this repository contains the full copyright notices and license terms. -->
     <group id="frequency" col="-1">
         <field name="frequency"/>
         <label name="weekday"/>
-        <field name="weekday"/>
+        <field name="weekday" widget="selection"/>
         <label name="day"/>
         <field name="day"/>
     </group>