[mlir][sparse] recognize NVidia 2:4 type for matmul (#76758)

This removes the temporary DENSE24 attribute and replaces it with proper
recognition of dense to 24 conversion. The compressionh will be
performed on the device prior to performing the matrix mult. Note that
we no longer need to start with the linalg version, we can lift this to
the proper named linalg op. Also renames some files into more consistent
names.