schema31: remove depreciated schema 31

[tmorrell]

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Description

Schema 3.1 has been depreciated by DataCite. There is no need for us to support it in new versions of this package. #82

Checklist

Ticks in all boxes and 🟢 on all GitHub actions status checks are required to merge:

• I'm aware of the code of conduct.
• I've created logical separate commits and followed the commit message format.
• I've added relevant test cases.
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• I've marked translation strings.
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