MDL-47062 Grades: Add an upgrade step to warn about changes in upgrade.

Puts a notice on the grader report about the change in aggregation method
from "Sum of grades" to "Natural".

Part of: MDL-46576
MDL-47520 core_grades: added CSS classes to the setup page

Part of: MDL-46576
MDL-46576 core_grades: Update upgrade scripts new version number