rename "Kotlin Trait" file template to "Kotlin Interface"

idea/resources/fileTemplates/internal/Kotlin Interface.kt.ft

@@ -2,5 +2,5 @@
 
 #end
 #parse("File Header.java")
-public trait ${NAME} {
+public interface ${NAME} {
 }

idea/resources/fileTemplates/internal/Kotlin Interface.kt.html

@@ -5,7 +5,7 @@
     <td colspan="3">
       <font face="verdana" size="-1">
         This is a built-in template used by <b>IDEA</b> each time you create a
-        Kotlin trait
+        Kotlin interface
       </font>
     </td>
   </tr>