Use `cache.identify` instead of manually creating a cache ID

client/src/components/toggle-menu/ToggleSolutionsForModule.vue

@@ -55,13 +55,11 @@
           },
           update(store, {data: {updateSolutionVisibility: {success, solutionsEnabled}}}) {
             if (success) {
-              // according to the documentation, this should be
-              // const id = `ModuleNode:${slug}`;
-              // see: https://www.apollographql.com/docs/react/caching/cache-interaction/#readfragment
-              // but in the actual cache, it's found like this
-              const id = `ModuleNode:{"slug":"${slug}"}`;
-              // there's an open issue on Stackoverflow here
-              // https://stackoverflow.com/questions/70264586/apollo-client-readfragment-with-custom-id-keyfields
+              // https://www.apollographql.com/docs/react/caching/cache-interaction/#obtaining-an-objects-cache-id
+              const id = store.identify({
+                __typename: 'ModuleNode',
+                slug
+              });
               const fragment = MODULE_FRAGMENT;
               const module = store.readFragment({fragment, id});
               const data = {