migrate from `.zenodo.json` to `CITATION.cff`

[sappelhoff]

similar to here:

• https://github.com/mne-tools/mne-bids/pull/1037

this has several advantages:

• unlike for .zenodo.json there is dedicated software to validate CITATION.cff
• we retain the same features
• we get a nice "cite as" ... button on the repository landing page

[sappelhoff]

done in https://github.com/sappelhoff/pyprep/commit/670c9831eeb62d711a7965a416c837d0567ed6ed