Nether and End dimension folders still use their old names

The folder corresponding to the Nether is called DIM-1 and the End DIM1, despite this not making much sense:

• Numerical dimension IDs were deprecated in 1.13, over three years ago (17w47a)
• It is not immediately obvious what these folders actually correspond to without prior knowledge
• Dimensions now use namespaced IDs literally everywhere else
• 1.16 custom dimensions do not use folder names like these

These folders should be renamed to something more intuitive and less reliant on ancient, deprecated formats, for example "the_nether" and "the_end", or even "dimensions/minecraft/the_nether" and "dimensions/minecraft/the_end" for consistency with data pack dimensions since they have no right not to be.