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

lms/djangoapps/instructor_task/tests/test_tasks_helper.py

@@ -371,7 +371,7 @@ class TestInstructorGradeReport(InstructorGradeReportTestCase):
 
     @ddt.data(
         (ModuleStoreEnum.Type.mongo, 4, 47),
-        (ModuleStoreEnum.Type.split, 3, 47),
+        (ModuleStoreEnum.Type.split, 2, 48),
     )
     @ddt.unpack
     def test_query_counts(self, store_type, mongo_count, expected_query_count):