xref gh-14142 (issue) and gh-14994 (always warn when saving an array with metadata).

Dtypes with metadata, or with subdtypes with metadata, either discard the metadata when saving via np.save, or crash when loading them via np.load.

Currently the "fix" adopted in 1.18 is to always warn.

EDIT (seberg): Giving the warning message:

metadata on a dtype may be saved or ignored, but will raise if saved when read. Use another form of storage.