Corrected migration SQL

"key" is a keyword for MySQL. This was missed due to tests being run against SQLite.

ECOM-5195

course_discovery/apps/course_metadata/migrations/0022_remove_duplicate_courses.py

@@ -12,12 +12,12 @@ def remove_duplicate_courses(apps, schema_editor):
                                     '  course_metadata_course AS c'
                                     '  JOIN ('
                                     '    SELECT'
-                                    '      LOWER(key) AS key,'
+                                    '      LOWER(`key`) AS `key`,'
                                     '      COUNT(1)'
                                     '    FROM'
                                     '      course_metadata_course'
                                     '    GROUP BY'
-                                    '      LOWER(key)'
+                                    '      LOWER(`key`)'
                                     '    HAVING'
                                     '      COUNT(1) > 1'
                                     '  ) AS dupes ON dupes.key = LOWER(c.key)')