[xylon] Legacy Libraries DEPR: Total deletion

[kdmccormick]

This can be done after the Xylon cutoff, as early as May 1, 2027.

This remove the ability for operators to migrate legacy libraries to V2 libraries!
Why do we need to do this at all? -> In order to remove MongoDB as a platform dependency and simplify the course delivery subsystem.

Steps:

• Figure out a way to retain legacy "library defaults" so that LegacyLibraryContentBlock children continue to render correctly forever.
• Remove LibraryRoot xblock
• Remove legacy library storage from modulestore
• Remove remaining legacy library APIs
• Remove all parts of the LegacyLibraryContentBlock (V1) (except that which would allow a migration to Problem Bank (V2)? TBD below)
• Remove studio_post_duplicate XBlock hook method #35192
• Grep for references to LibraryLocator and LibraryBlockUsageLocator. Destroy as much of the logic as possible. If possible, actually remove those key classes from OpaqueKeys.
• Generally grep for "libraries" and remove any legacy library logic you an find.

What must continue to work when this is done?

• Existing LegacyLibraryContentBlocks should continue working from a student's perspective, forever. Essentially as dumb containers of randomized blocks.
  • TBD: Do we allow migrations to ProblemBanks forever?