feat: Read course indexes from MySQL, not MongoDB (#29184)

Description
This is a follow up to #29058 and #29413. This is the next step in moving part of the modulestore data (the course indexes / "active versions" table) from MongoDB to MySQL.

There are four steps planned in moving course index data to MySQL:

Step 1: create the tables in MySQL, start writing to MySQL + MongoDB ✅ done
Step 2: migrate all remaining courses to MySQL ✅ done
Step 3: switch reads from MongoDB to MySQL (this PR)
Step 4 (much later, once we know this is working well): stop writing to MongoDB altogether.
Supporting information
OpenCraft Jira ticket: MNG-2557

Status
✅ Tested with a large Open edX instance is in progress.

Testing instructions
Try making changes in Studio and verify that they work fine.

Deadline
None

cms/djangoapps/contentstore/tests/test_course_listing.py

@@ -163,8 +163,8 @@ class TestCourseListing(ModuleStoreTestCase):
             self.assertEqual(len(list(courses_iter)), 0)
 
     @ddt.data(
-        (ModuleStoreEnum.Type.split, 3),
-        (ModuleStoreEnum.Type.mongo, 2)
+        (ModuleStoreEnum.Type.split, 2),
+        (ModuleStoreEnum.Type.mongo, 1)
     )
     @ddt.unpack
     def test_staff_course_listing(self, default_store, mongo_calls):

cms/djangoapps/contentstore/views/tests/test_course_index.py

@@ -403,13 +403,13 @@ class TestCourseIndexArchived(CourseTestCase):
 
     @ddt.data(
         # Staff user has course staff access
-        (True, 'staff', None, 1, 19),
-        (False, 'staff', None, 1, 19),
+        (True, 'staff', None, 0, 20),
+        (False, 'staff', None, 0, 20),
         # Base user has global staff access
-        (True, 'user', ORG, 3, 18),
-        (False, 'user', ORG, 3, 18),
-        (True, 'user', None, 3, 18),
-        (False, 'user', None, 3, 18),
+        (True, 'user', ORG, 1, 20),
+        (False, 'user', ORG, 1, 20),
+        (True, 'user', None, 1, 20),
+        (False, 'user', None, 1, 20),
     )
     @ddt.unpack
     def test_separate_archived_courses(self, separate_archived_courses, username, org, mongo_queries, sql_queries):

cms/djangoapps/contentstore/views/tests/test_item.py

@@ -2560,7 +2560,7 @@ class TestXBlockInfo(ItemTest):
         self.validate_course_xblock_info(json_response, course_outline=True)
 
     @ddt.data(
-        (ModuleStoreEnum.Type.split, 4, 4),
+        (ModuleStoreEnum.Type.split, 3, 3),
         (ModuleStoreEnum.Type.mongo, 5, 7),
     )
     @ddt.unpack