DOC: which versions support B0Field* metadata

[bpinsard]

As datasets creators migrate to the new BIDS B0Field* fieldmap intent specification, it might be good to mention in the doc, which version supports it or not, including retrospectively for LTS versions. Otherwise, one might run fMRIPrep thinking it will correct distortions, while it doesn't. From what I found-out, it's only from 21.x that it's supported.
related to #2560