MNIST imports broken in neural networks notebook

[DrJonnyT]

The very first line no longer works: using Flux, Flux.Data.MNIST gives the error UndefVarError: `MNIST` not defined

The MNIST dataset is now available from MLDatasets, but it seems like the implementation is different so it's not just a straight swap This is in Julia 1.9.2, with Flux v0.14.5