changed insert to insert_one (#23908)

I wanted to make a byte-sized contribution but there were no Jira tickets so we decided, thanks to a conversation with @jmbowman through the Open Edx Community #incr (Slack) channel, to collaborate in the elimination of warnings listed in the Warnings Report at https://build.testeng.edx.org/job/edx-platform-python-pipeline-master/warning_5freport_5fall_2ehtml/

This PR contributes to the elimination of deprecation warnings, specifically the one mentioned above and reported in the Warnings Report.

Changed insert to insert_one in the following file:

* cms/djangoapps/contentstore/management/commands/tests/test_cleanup_assets.py

This warning occurs due to deprecation: https://api.mongodb.com/python/current/api/pymongo/collection.html#pymongo.collection.Collection.insert

cms/djangoapps/contentstore/management/commands/tests/test_cleanup_assets.py

@@ -64,9 +64,9 @@ class ExportAllCourses(ModuleStoreTestCase):
         query['_id.name'] = all_assets[0]['_id']['name']
         asset_doc = self.content_store.fs_files.find_one(query)
         asset_doc['_id']['name'] = u'._example_test.txt'
-        self.content_store.fs_files.insert(asset_doc)
+        self.content_store.fs_files.insert_one(asset_doc)
         asset_doc['_id']['name'] = u'.DS_Store'
-        self.content_store.fs_files.insert(asset_doc)
+        self.content_store.fs_files.insert_one(asset_doc)
 
         # check that now course has four assets
         all_assets, count = self.content_store.get_all_content_for_course(course.id)