When upgrading a world from before 24w21a, structures saved by structure blocks are not accessible anymore.

How to reproduce

1. Open a world in 1.20.6 or 23w20a (or any other version)
2. Use a structure block to save a structure
3. Verify that you can load the structure using a structure block or the /place template command
4. Update to 24w21b
5. Try to load the structure using a structure block or the /place template command
6. The structure doesn't exist

Cause

24w21a changes the folder of structures from structures to structure. This change also affects the generated folder in a world. (The changelog only mentions this change for data pack content, not for the generated folder.) When upgrading a world, the saved structures do not get moved to the new folder.