add `@allow_nobody` to `@get /semester/:timestamp/week`, fixes #1598

Closes #1598

Merge request studip/studip!1028

app/routes/Semester.php

@@ -46,6 +46,7 @@ class Semester extends \RESTAPI\RouteMap
      * Returns the semester week as string for a given string
      *
      * @get /semester/:timestamp/week
+     * @allow_nobody
      */
     public function getSemesterWeek(int $timestamp)
     {